MG-Cars.com
MG MGA Can't get the MGA past 3000 RPM BBS discussion at MG-Cars.net
MG-Cars.net
Welcome to our resource for MG Car Information.
Recommendations
Parts
MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.
|
|
MG MGA - Can't get the MGA past 3000 RPM
| This is a total rebuild of an MGA Coupe 1500. engine was professsionally rebuilt as well as the carbs. My problem is that with the car idling ,in the driveway I am able to rev the engine past the 3000 RPM mark with no trouble. The problem starts when I take it out on the road. It runs well at lower speeds and RPM's but the moment I try to push it past the 2500-3000 RPM mark ,the car bogs down and will not surge or run faster. If I nurse it slowly or slightly lift my foot off the gas pedal the car will pick up speed a little.It would take me app a mile to get the car from 2800 RPM's to 3000 RPM's and this is by feathering the gas pedal. Anybody got any ideas. My checking of the ignition shows what seems to be proper timing and advance, but I think that the weights inside the dizzy might be installed wrong and this is not creating enough advance at higher speeds. As they say most carb problems are ignitions problems. Is this my case? Gordon |
| Gordon Harrison |
| I don't think this is a carb problem mascarading as an ignition problem. No this sounds like a classic case of not enough fuel. Thus yes it is either a carb problem or more likely a fuel pump problem. |
| Bob (robert) yes Y8 is toast again :) |
| Is the problem the same in any gear? Does the RPM limit change depending on gear? I Had a VW once that would not get above 25mph no matter what you did while driving. Under no load, it would rev fine. A smiple tune-up cured it. I would replace the points and plugs first. Then look at the distributor weights if necessary. Chuck |
| Chuck Schaefer |
| Gordon to help you understand this problem better, if it were ignition related then the problem would cause a lot of heat (advance/retarded). I doubt if that is the case and thus it is fuel related. |
| Bob (robert) yes Y8 is toast again :) |
| Fuel pump is new and I just had to replace the float bowl seals due to leakage so I have fuel ,in sufficient quantity in the carbs. The problem is consistant with any gear and the plugs /points/condenser are all new. There does not seem to any excessive heat. Tune the carbs rich or set them lean does not seem to help. ONE thing thought. While tuning the rear carb with my UNISYN I find that when I place the US oer the rear carb, the engine idle slowwly reduces to the point that I must remove the US or the car would die. This seems to me to lack of air flow yet the carb pistons rise and fall freely. Gordon |
| Gordon Harrison |
| Gordon;; I know it sounds funny but check the linkage between the two carbs. It is possible that only one of the two is opening up. This will give you a good idle but now power when asked for.Also run a timing light on it and check the timing at 3000 rpm. maybe the advance weights are stuck. take off the piston dampers and put a small stick or soda straw in each and see if they both rise equally when you blip the throttle. Please keep us advised when it's cured. Are you considering going to the Ontario Goof in Kingston? Sandy Sanders |
| Sandy Sanders |
| It is most likely a fuel feed problem. Disconnect fuel hose from rear carb, hold in a catch pan, switch on ignition, observe fuel flow. You should get a gusher, at least one pint per minute. If you only get a trickle, look for a kink in the steel fuel pipe, or a clogged filter screen at the pump inlet fitting, or some obstruction at the tank picup pipe. If you get a gusher, then look for obstruction in the carburetors, possibly obstruction at the inlet filter sceen, junk in the bottom of the float chamber, decomposed rubber grommets between teh float chamber and carb body obstructing flow through the banjo bolt, or a flow obstruction around the main jet seals. |
| Barney Gaylord |
| Sandy, I tried the trick with the piston dampers any they are OK. Linkage between both carbs is tight. Do not forget that the carbs were professionally rebuilt and the throttle arms bushed. The car starts on a dime and idles well. It's just so frustrating not to be able to go past 2500/2800 RPM. There is a complany up here called Zekes Performance. They have a sun dizzy machine and I am really thinking about pulling the dizzy and have them give it a spin. Even if the dizzy is Ok it would certainly eliminate it from the problem. Did not know there was a GOF in Kingston. When? Gordon |
| Gordon Harrison |
| Barney, I will do everything you say tonight and report back. After my last problem you helped me solve I hope you can get this one fixed. While I have your attention I was looking in my reference papers to see if I could find some help with this problem when I came across a five page letter from Andrew Rich in regards to the kevlar Composite Body MGA. I was thinking buying a body to put on my Coupe but then decided to restore it.The letter is hand signed by Andrew and are in great condition. In these five pages are all of the pricing /tech details and explanations of the car. Interesting ??? Gordon Would you have an email addy for Mr. RIch ? Gordon |
| Gordon Harrison |
| I had the same problem last year, it turned out to be the float bowl seals, there were two sets in each float bowl, when the union was tightened down it compressed the seals and they blocked off the feul lines. I would bet something is wrong with those seals. |
| The Wiz |
| Check the dashpot oil in the carbs, if they are empty it can also cause this problem. If your rebuilder shipped them to you they would have been shipped without oil. |
| John H |
| The Wiz, Funny but I just changed the float bowl seals because they were leaking. Is it possible to tighten them to tight. They ones I removed had a compressed indent on the inside around the part that sticks up. John, The carbs have dashpot oil supplied by LBC Car Company. Should be the right type and they are topped up. So far looks like I may have fuel starvation. Tonight will tell when I can get a look at the float bowls. Gorodn |
| Gordon Harrison |
| Pure fuel starvation would allow you to get up to speed and then drop off power and speed as the fuel is used up. I think that the fact that the problem is independent of the gear chosen, and consequently load, points to ignition issues. My money is on the ignition as the root cause. Since I'm guessing, I think that the points being barely opened. Chuck |
| Chuck Schaefer |
| Review: Rev's great sitting in the driveway. 4500 RPM's easy to reach with a flick of the gas pedal. Runs well at lower RPM and speed around town. Pulls well from a stop sign and cruises smoothly as long as I stay under 2500 RPM's. No matter which gear chosen, bogs down as I near the 2800/3000 RPM level. Car will reach higher(just) speeds if the gas pedal is slowly feathered but takes forever and never gets above 3200 RPM's. At 2800 RPM, on the road, the car will bog down if I step on the gas but surges forward SLIGHTLY as I let go of the gas pedal. Starts on a dime and idles well. Changing mixture, rich or lean seems to do nothing. Engine and carbs are professionally rebuilt. Timing seems to be accurate, to my level of competence. Unknow factors are dizzy internal weights and centrificual advance unit. AND FINALLY, the old saying goes....MOST CARBS PROBLEMS ARE IGNITION PROBLEMS. Gordon |
| Gordon Harrison |
| The reason why this is most likely fuel supply related is very simple to understand. Fuel requirements are directly proportional to power. Which in turn is directly (almost) proportional to RPM. Thus as the engine begins to rev and the power increases then more fuel is required to produce that power. Your engine is not getting it. You have noticed that feathering the throttle gives you a little more speed/power. That is exactly what you would expect due to the engine being more efficient with the fuel available. If you totally disagree with these observations and insist upon the dissy then simply remove the vac tube, set your timing light on and rev the engine,Just note that the timing advances somewhat due to centrafuge. Good luck. |
| Bob (robert) yes Y8 is toast again :) |
| Flexible fuel pipe cavitation? Steve |
| Steve Gyles |
| I had a a similar problem with a newly rebuilt engine and carbs. It turned out to be a bad valve in the rear carb, causing fuel starvation under load. In effect, I was running on two cylinders. The rubber in the needle/seat valve had deteriorated and caused the valve to stick. This happened after about 6 months with the new engine/carbs. Good luck. M.D. '57 Coupe |
| M. D. |
| Put a tee in the fuel line before the carbs, connect to a common fuel pressure gauge with a long hose that will reach in to the car. Drive and observe fuel pressure - it should stay constant somewhere around 2psi up to redline. The old Petroflex lines at the tank to pump can develop pinholes that don't leakl gas and do pull air, and you can't see that they are bads. Any leak before the pump can cause this; any leak after the pump will not, and will be visible since gas will come out. Another possibility is sticky points, or points with weak springs - a common problem. Points bounce or float is a prime cause of this kind of problem. It should take a minimum of 22 oz to pull the points open, and up to 32oz is better. Measure at the contact with a spring scale, or you can make do with weights hung from a string with a hook on the points. The engine is operating at maximum efficiency around 3000, and the load on the ignition is also maximum here. Bad cap, rotor, plugs or wires can do it. Coil trouble may show up here or at higher speeds under WOT. FRM |
| FR Millmore |
| Hi Gordon I created similar effect by setting float levels too low gordon |
| g c pugh |
| Sounds just like my car after a 3 month voyage from the US to Japan. Idled and ran fine, but when you would try to drive it, it had no power. I refused to believe that it was a fuel problem as it acted like an ignition problem. ALl ignition components checked out fine. I finally discovered that the problem was that the cork jet seals had dried out, leaving in effect, no seal. Got some new seals, soaked them in engine oil overnight, re-assembled everything, pre-set the carbs and she started right up. After following Les's carb tuning advice, the car has been fine now for three years. Good luck! |
| Tommy |
| had exact same problem, was caused by faulty SU electronic fuel pump, replaced under warranty |
| Cameron McIntosh |
| You could test the fuel starvation theory by taking the car out and runnin it up to speed and when it stalls out turn the ignition off, put clutch in and place in neutral. (Not recommended on an interstate highway.) Coast to a stop and check both float bowls. If one or both are low, you have fuel starvation. Then you can sort out why. Stuck needle? Clogged filter? Pump not pumping? |
| Steve |
| To logically approach this problem, we should take note of the engine dying when the Uni-Syn is used on one carb. That's the only one doing any work, I think. When the US is used on the other one, no difference. Having never used a Uni-Syn, I don't know much about them. And they shouldn't restrict the airflow too much, I guess. But with only one carb (getting enough gas?) I suppose if you took away even a little airflow from the other one (that is working), the engine would starve for enough air to run well. After replacing the float bowl lid seals, did you reset the float level? |
| Tom |
| There's a big difference between carbs working under no load, and working under road conditions. I'd basically start the setup process from the beginning, and make sure that everything is done correctly step by step, setting the mixture on each carb, setting the balance of teh carbs, setting the timing too. Something is out of spec here, and it's quicker to start from first principles than to test different theories. Once it is set up correctly, problems should either dosappear or be much easier to localise. |
| dominic clancy |
| Gordon If the pump is only slowly pumping try getting up to speed and then coasting (clutch in) for a few seconds to re-fill the bowls, then clutch out and floor it. My fuel pump also gave up the ghost over a few weeks and I suffered from such a starvation problem. It was easy to hear when I removed the cover over the diff as the pump would stop for a while and then rattle into life again (sometimes aided by a shorp tap from a spanner!!). Have you lubed the dizzy? I know it is left off many peoples check list but the vac advance mechanism will seize up if not lubricated. Mine (originally from Texas) was seized solid and required rebuilding before it would work. Hope you sort the problem, let us know. Neil |
| Neil Purves |
| My imopression is that the floats were not set high enough in the bowls. From memory, I think that a 7/16 rod should be inserted under the hinged lever for proper adjustment. Setting the floats too low does not let enough fuel accumulate in the bowls and the carbs starve under high demand. Karl |
| Karl Leclerc |
| Bob (robert) Barney & Steve, I pulled the fuel bowls off the car last night and did a complete cleaning. Installed new bottom bowl seals...cleaned out the fuel bowls and checked the operation of the gross jets. Everything appeared OK.I disconnected the fuel line to the rear carb and checked the flow,,,very good flow. M.D & Tommy Not to sure, at this stage if I want to be pulling apart the jets and needles. Will keep it in mind for the last shot at solving the problem. This is only due to a mild fear that the more I take things apart,the more I can screw things up. F.R Millmore I am kind of leading towards your line of thinking. I checked the points, and the gap is correct. I don't think it is that but I am leaning toward centrifical advance or mechanical advance weights inside the dizzy. g.c. Pugh I run with gross jets and they seem to be functioning properly. How high should the float bowl be (in the carb bowl) when I remove the cover? Mine were sitting with the top off the float bowl about 2/5 down from the top of the rim of the float bowl. Cameron Due to the good fuel supply and that the pump is brand new I am going to eliminate it as a possibility for now. Steve, Will try your test out and see what happens. Dominic Did the top to bottom tune-up last Sunday.Everything seemed OK except for the rear carb dying slowly when I moved the UniSyn onto it. Neil I would be real interested to learn how to lubricate the vacuum advance mechanism. I am not sure what you mean. I know that you should put oil on the screw just under the rotor to help lubricate but could you explain further what you mean. Karl Is the float level set the same way if you are using Gross Jet and not needles. Finally I spent the rest of the evening last night reading up on my dizzy and basking in the fumes of gasoline radiating off of my hands. I am strongly drawn to the theory that it is a problem with my dizzy. I have a spare one, so this weekend I am going to set it up with new points and such...install it and see where this leads me. I will do this before taking apart my carbs beacuse I am more comfortable playing with the dizzy than the carbs. I will let everyone know what happens. A GREAT BIG THANK YOU TO EVERYONE> Gordon |
| Gordon Harrison |
| Gordon Whilst I am with the group that favours a fuel problem (not normal for me, I usually go with electrics), I did have a problem with my dizzy that I have already explained in one of the other very recent threads "Why was the coil moved?" My dizzy came back from refurbishing with the wrong mechanical advance cam. If I set the dizzy up to the correct static advance it caused the engine to run flat above about 2500RPM . How have you timed the dizzy - statically or dynamically? Steve |
| Steve Gyles |
| Ok...I will take a shot at this. You say the motor was professionally rebuilt. What are the chances the cam timing in relation to the crank was set incorrectly? I would think if there was not enough fuel at a higher rpm..and the motor was running lean on one carb or the float level was wrong, you would have some missfiring from a lean mixture. |
| Ray Ammeter |
| Gordon I think Cameron is right. I once had this problem with a coupe. It turned out that it had a non-original fuel pump that was mounted in the boot. When I replaced it with an orginal pump (and mounted at the chassis crosmember) the problem was gone. Hope this helps Rutger www.rutgerbooy.nl |
| Rutger Booy |
| Gordon the pump was brand new SU, the electronic version without points, purchased for the restoration. The same supplier had also completely rebuilt the carbs, a beautiful job. The car behaved as you describe and I was asking the supplier about the possibility of vapour lock or vibration issues when the faulty pump issue came up. There was a faulty batch of pumps but since we don't all fit them straight away after purchase, the problems aren't always immediately evident. If the supplier had not told me of the problem it would have taken a very long time to find. The pump appeared to be perfect with good flow and a satisfying tick. It always worked when the ignition was turned on but failed to deliver enough fuel under load. I wasted a lot of time on coil, timing, dizzy, filters, heat, vibration etc. I know that its not much fun swapping a pump on a coupe but it is an easy check. Good Luck Cameron |
| Cameron McIntosh |
| Hi Gordon your float levels sound about right, I did get rid of the grose needles on my car on the advice of my local mg specialist as they had some problems with them in the past, I went back to a slightly modified version of the original needles from Burlen. gordon |
| g c pugh |
| Gordon; Have you removed the pistons and chambers to check the actual fuel level at the bridge, Do this with the key turned on and looking down the hole at the moveable jet the fuel should be at approx 1/16" below the top.the fuel level is not only controled by the float level but by the thickness of the washers between the float chamber and the bottom of the carbs. wrong thickness gaskets at this point can create all sorts of problems such as you have with the Uni-syn causing one carb to run lean when you try to check it. Sandy Sanders |
| Sandy Sanders |
| Don't think it is too likely with the design of the standard MGA fuel cap, but I have seen other vented fuel caps get partially clogged. This can create a vacuum in the fuel tank and limit fuel delivery at higher speeds. Anyway, it is easy to check by running with the fuel cap loose and see if the symptoms change. A partially clogged inlet screen on the fuel pump could cause the same symptoms. |
| Jeff Schultz |
| Have you pulled your plugs and checked their color to see if they are receiving the same fuel/air mixture? If one carb was doing all the work as someone suggested or if one's leaning out that might give you a clue as to which carb was the likely culprit. When the car bogs, what happens when you just hold the pedal down? Will it eventually miss or die out? Why can't you use an advance curve timing light to check your curve at higher rpms? Set it to 32 degrees and rev the motor up to 3000rpm and see what you get. That should tell you if your advance weights are set properly, yes? Who did your dizzy and why do you think the weights were installed wrong? There's pretty much only one way it goes together and if you goof that then it won't work at all, so if you disconnect your vacuum line and check your mechanical advance at 3000rpm, that should settle that question. The only time I had a problem like this that WASN'T fuel starvation, it turned out I needed a valve job. The car ran fine at lower speeds, etc., but would bog down and lose power at the higher end of all gears. That's why I asked if your engine misses- with fuel starvation you can usually push it to the point of dying out. Mark |
| Mark |
| Check that the carb pistons are moving freely up and down. Your problem seems to fit the symptoms of a sticking piston. This is usually caused by a mis-aligned or bent needle or a high spot in the cover/dashpot. Malcolm |
| Malcolm Asquith |
| Gordon- Do as I suggested above with the pressure gauge. That will definitively tell you if the problem is fuel supply related and before the carbs, and it is the only way to do it under load. Takes 10 minutes and then you know where to look. The Uni-syn deal means that your throttle synch at idle is off, but that won't cause the no-go past 3000. Sticky points may limit the engine speed as I said, but that usually also gives severe miss and break up as it does so. Bad/sticky advance throws the timing off and will cause poor results, but not a definite shutoff as you have. FRM |
| FR Millmore |
| My goodness, So many helpfull people and all with my MG in mind. THANK YOU. As stated this weekend will be one of testing. If the tests are within in the circle of my ability and my tools I intend to try every test that has been offerred. I will document the results and get back to everyone. I will start with a valve adjustment check (as they must be set cold) and then a compression check. This will be followed by a complete tune-up and then the testing will begin. Stay tuned for the results. Many thanks Gordon |
| Gordon Harrison |
| Hi Gordon. It sounds like fuel starvation to me also, but I have had similar symptoms caused by defective ignition parts including bad points, condensor, and ignition coil. Please let us know what you find. Good hunting, Glenn |
| Glenn |
| Hello Gordon. Everyone seems to have covered most possible reasons for your problem. I was going to suggest the cap venting, but that has been mentioned also. But I do have another 2 cents to add - have you checked the flow through the exhaust/silencer? From my motor cycle days, I remember how a blocked exhaust caused a lack of power. Good luck, and keep us informed once it is sorted. Peter. PS Valve clearances are usually done hot, I believe, set at 17 thou. |
| P. Tilbury |
| P. Tilbury I recall reading in a recent archive that someone was having the same difficulty as I and it was the baffles inside his muffler that were loose. When at low speed, the baffles did not interfer with the exhaust flow yet at high speed the higher exhaust pressure displaced the baffles enough to block the exhaust flow therefore choking the engine. One thing interesting about all of the above help is that all the parts are new. NEW ENGINE NEW MUFFLER NEW CARBS NEW FUEL PUMP NEW POINTS CAP CONDENSER PLUG WIRES ETC The only old part is the dizzy bodyand this is the one part that has not been touch PROFESSIONALLY. My money is on the distributor,and we shall find out this weekend when I change it. PS: My book says valves cold at .15 (same as hot at 17 I guess) thank you. Glenn,I really don't care what the problems is as long as I can get it fixed and then DRIVE...DRIVE...DRIVE. Thanks Gordon |
| Gordon Harrison |
| Glenn has his rogue browser on line tonight!! Has the snow gone yet Glenn? Steve |
| Steve Gyles |
| Hi Steve. Yes, the snow has mercifully departed, and we now have nice warm, MG driving weather. I have my MG out of storage now, and have been indulging at every oppoortunity. How is your beast running, and how is the weather, besides rainy.....Glenn |
| Glenn |
| Gordon, I'm certain you are on the right track in spite of the other advice. However, I've been wrong before and I'll be wrong again. That being said, don't discount anything. Some bad parts do get out in the marketplace in spite of the best manufaturing practices, testing and inspection. Even professional mechanics do it wrong once in a while. Just bacause all of those things stated are new or refured professionally, don't assume that it is correct. I'm looking forward to hearing what you find is the culprit. More importantly, wish I could see the grin on your face when you finally identify it and get the car running right. Chuck |
| Chuck Schaefer |
| Your problem sounds like what I had when my coil was failing. Idle was fine, no load was fine but no acceleration in gear. I see that you have lots of new parts, but that does not mean they can't fail. |
| Dan Smithers |
| This thread has been timely for me. I finshed my MGA restoration last month & have been sorting out the last few details and breaking in the engine. The engine was completely rebuilt as were the carbs & I have just completed a complete restoration of an MGA including the engine, carbs & distributor. After firing it up and completing the initial run in I decided to fine tune the carbs and and check timing, tappet clearance and points before going further. The engine ran terribly after all my efforts. I checked everything & was at my wits end when I checked into our site and found this thread. I had tightened the butterfly shaft bolts on the carbs but not tight enough so I was running on one carb. Thinking back on it it ran surprizingly well on one carb.Thanks to Sandy for the throttle linkage idea. I had a similar problem back in 1959 and it turned out to be the debrie from a disintegating rubber seal at the base of the carb where the float bowl attaches. The only difference was that the cut out or flat spot hit around 4000 rpm. Cheers Morris |
| M. E. WADDS |
| Gordon, I've also experienced baffle problems restricting the exhaust flow. See if you can push a flexible wire up the silencer to check for any obstruction. If when setting up the engine you've had a lot of backfiring in the silencer then this can easily cause damage to the baffles even when new. In my case some of the fibre glass was partially blocking the main through pipe. |
| J H Cole |
| OK Where do I start. First the two problems, I had planned to install my second dizzy but found the terminal wire needed replacing. I bought the new wire. Apon trying to install it it proved to be TOO SHORT. Of course the parts store is now closed. So I went with plan TWO. Complete fuel flow test tank to carbs...plenty of gas Float bowl inspection and cleaning.....OK Checked float bowl level....OK Checked Gross Jets....OK Topped up dash pot oil....OK Lubed dizzy....OK Checked Vac advance......OK Checked carb linkage ....Tight and OK Check fuel cap seal.... no pressure build up Adjusted the valves......OK Complete compression test....140 all 4 Re-set the points....gap too wide Checked the dwell angle....was 48 reset to 64 Static timed the dizzy.....got it timed OK MY problem is getting the point gap set so that the dwell angle is correctly set a 60 degrees. Giving me a hard time but I am close. With all of the above I noticed a remarkable differnce in the car. Althought it will now head near 3500 RPM and is quieter...peppyer...smoother, I still need to get the points and dwell set so that I can then proceed to the carb tune up. If I continue to follow all of the advise offerred I am sure it should start running properly. Still left to do. Coil check Points and dwell Carb tune up Exhaust check It got to be late and it was MOTHERS DAY so I had to shut down. The car is running better and I see a combination of problems seem to be the culprit. It's getting there. Once agin THANKS EVERYONE. Gordon PS: FR. I am going to try your pressure test but at the moment I do not have a pressure guage in my tool kit. Even when it is running right I will check it. |
| Gordon Harrison |
| Gordon- Congrats on progress, it is always best to get the easy stuff first; the hard stuff frequently disappears along the way! The gauge you need is the common Vacuum/pressure one sold for tuneup work, which you should really have anyway; goes from 10psi - zero - 28" vac. FRM |
| FR Millmore |
| Gordon Don't forget the silly gotchas like the one that had me puzzled for a month or 2. Make sure you have the rear air filter on the right way up or you negate all your carb balancing work and end up effectively running on one carb. i.e. You block off the air balancing hole to the piston. Apparently this is quite common. Bob West informed me that about 50% of the customers that come to him with 'carb' problems have the rear filter box on upside down. Steve |
| Steve Gyles |
| Steven, I just had the rear air filter off....how is it possible to have it upside down? Thanks |
| T McCarthy |
| Extremely easy. Rotate it clockwise or anti-clockwise 180 degrees and you will see that the offset hole on the the carb flange mating face will be blanked off. Steve |
| Steve Gyles |
| Re my last post. I have taken a few photos of the air filter to show the problem: http://www.mgaroadster.co.uk/hot_press.htm Steve |
| Steve Gyles |
| WOW, color me a dope! Thanks for that tid bit. It may make all the difference. |
| T McCarthy |
| Tuesday morning, Some more fine tuning and faster she goes. Actually had it out on the highway pushing 3500 RPM but the carbs started to act up. I now need to fine tune the carbs. Presently timing is slightly advanced and the points are still giving me a hard time being set at .015. I think there may be some free play in the dizzy so this weekend I am trying my backup one. With the carbs fine tuned and new air filters I am pretty sure I can make it run like it should. Many thanks to every one for their help. Gordon |
| Gordon Harrison |
| You learn something new every day on this website. I checked my rear carb air box and found it to be blocked. The same with the front. What has happened is that the wrong gasket was used and had no holes for the piston to be in balance. So a few holes later and the car has more power but backfires untill it warms up. What can I do to get this corrected? Thanks, Neal Turner |
| Neal Turner |
| Neil I am guessing that for starters, with the pistons now working correctly, you may need to adjust your mixture. Also check, as mentioned by others above, that the dampers have oil in them. Steve |
| Steve Gyles |
| Good thread! About the only thing not mentioned is the linkage that connects the two carbs. The nuts connecting the two portions of the linkage should be loosened while balancing the carbs and the re-tightened after. Missing either of these steps can cause problems similar to those experienced above.. Rich |
| Rich McKIe |
| Gord- I experienced a similar problem, however ,let me say that I have a MGB 1800 .060 over..with MGA manifold and Falcon stainless exhaust...could not pull past 3500. I put the header ,larger tailpipe and moss sports muffler on..no other changes...and it will pull to redline..I had perhaps too much backpressure and breathing problems,maybe because of the restricted exhaust...I also read that the MGA manifold can have a casting restriction near the "Y".. Just a thought.. John |
| John Coryea |
| John Most interesting. I also have an 18V with all the 1500 external bits and Falcon exhaust. Never had RPM limiting problems. Steve |
| Steve Gyles |
| John C- That's a typical result of blocked muffler from either disintegrating baffles, crushed pipe, or mouse nests. It's not the Falcon per se that fixed it, just a system with a big hole from end to end! FRM |
| FR Millmore |
| Steve, just checked my gaskets positioned between the air filter and carburetter and find that they are both blanking off the two air balancing holes. I remember a few years ago wondering what these are for and assumed they were redundant casting holes (theres one or more on the cylinder head like this). However I've made new gaskets with the extra holes and the engine seems more responsive , or am I just imagining it? Thanks for the timely observation |
| J H Cole |
| Neal & John You are confirming what Bob told me. I bet there are dozens of MGAs out there with the vents blanked off. It was the higher fuel consumption that I noticed, as much as performance. As I understand it, the pistons still work, but there is more lag than ideal. Steve |
| Steve Gyles |
| JH, You are right. There is more response. I have opened the balancing holes and feel more power, not much but you can tell that it is there. I have a feeling that you will be getting better mpg. I still do not know why this works. Thanks, Neal Turner |
| Neal Turner |
| Well, darn, I checked my air filters and, sure enough, the ports were blocked off. The filter bases were on correctly but the gaskets backwards. Does anyone have a description of exactly what these ports do? I've looked into all the SU stuff I have and while I see them in the pictures they are never referenced in the text. After unblocking the ports I find that the car is now running much leaner. To the point of not being able to hardly run at all without the mixture knob being pulled. So tonight I get to reset the jets. Another question... If anyone else has unblocked the ports was there a specific number of flats you needed to go to get the mixture back? Maybe I'll be lucky and if three out of four of you needed to go 3 flats down I'' start there and see how it goes. Dave |
| David Ahrendt |
| Update: Guess what ? Through my own stupidity and desire to protect my car from theft I added a fuel line cut off valve. This was inserted into the fuel line and as a result it's opening was smaller than the fuel line. Fuel line 1/4inch and valve 1/8inch. This caused fuel starvation at the higher RPM. My car can now reach these higher RPM but is still having a bogging down problem. Two things, I found the needles in the carbs are BB which is TWO needles LEANER than GS (stock needles). Also that my dashpot and needle carrier are binding ever so slightly. This would lead me to believe that the needle / needles are NOT rising properly upon accelleration and starving the carbs of gas. This coupled with the extra lean needles must be what is now causing my stumble upon stepping on the gas. I will change the needles and LIGHTLY clean the dashpots for a smoother slide and see what happens. I will keep everyone posted. |
| Gordon Harrison |
| David, I just opened the ports on my carb gaskets and found that the engine does require more warm-up than normal. After warm-up I find that I have more power and runs really well with the choke in all the way. I do use the choke more before warm-up than before. Thanks, Neal Turner |
| Neal Turner |
| Gordon- The fuel pressure test would have caught that immediately. Dave & Neal- The two ports allow the (near) atmospheric pressure inside the filters to reach the bottom of the pistons. The flow of air through the carb creates a vacuum that is communicated to the space above the pistons through two holes in the piston bottoms. Since the pressures are not balanced, the piston rises, adjusting the mixture in accord with airflow. For you young uns, that's an analog mass airflow meter. The rise of the piston is limited by its weight and the pressure from the big spring, and the rate of rise is controlled by the damper assembly. This gives enrichment on acceleration, just like an accelerator pump, but nicely matching the airflow rates, as opposed to the arbitrary slug of fuel from a pump. The mixture needles are calibrated with all this in mind, for each engine. Any change in any of these parameters calls for different needles. If the ports are blocked, the piston rises only as much as air leakage around the front lower section allows - slowly - thus giving very rich mixture and slow response. Once the piston manages to rise, it falls very slowly when the throttle is released, giving a very lean mixture, but that is not nearly as noticeable. FRM |
| FR Millmore |
| Dave Arhent, I've discovered from this board that setting the jet head (nozzle) .060" from the bridge is a very useful starting point for the fuel mixture. Tysen |
| T McCarthy |
This thread was discussed between 07/05/2007 and 30/05/2007
MG MGA index
This thread is from the archive. The Live MG MGA BBS is active now.