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MG MGA - Engine 'motorboats' at high rpm

Okay, I've been struggling with this one so I decided to break down and post it. I've had this problem since I installed my new head last year. The engine is an 1800 5-main bored +.030 with 9.5:1 compression. Stock cam grind. The head is an early B head which has been heavily ported and polished, with oversized stainless valves, bronze valve guides, and new valve springs.

The problem is this: at 4000 rpm and above, either at prolonged speed or sudden acceleration past that rpm, the engine loses power and "motorboats". If you're on the highway it feels slight, like driving into a bad headwind. If you give it wide open throttle the loss of power is very noticeable. The problem is the carbs are leaning out. It is not a fuel delivery problem- the float bowls have been checked when it occurs. The spark plugs show white. I can richen the mixture so that the problem disappears and the engine will run all day over 4000 rpm with lightly colored plugs, the problem is that then the mixture is way too rich at idle and loads up on the plugs and produces black exhaust.

The carbs are HS6, the exhaust is stock MGA. I've tried 3 different needle profiles: SY, RI, and BCA all with the same result- too rich at idle, too lean at speed. I'm using the lighter weight red springs in the carbs. I decided to measure how much lift I was getting in the carbuetor pistons by putting in an SU balancing flag and taping a ruler to the dashpot cover. At idle they're only showing 1/16" of lift, with an increase of 1/16" per every 500 rpm. Testing without springs is only slightly better and the results are the same in the front and rear carbs. Everyone including myself agrees this is not enough vacuum, but we can't figure out why.

The pistons move freely up and down and the drop test is good. Carbs were rebushed, cleaned, and rebuilt last year and mechanically they seem to be working great.

My thoughts so far are:

1). Stock exhaust too restrictive. Along with being the stock 1-1/2", a big rock put a big dent in the down pipe wich I'm sure has reduced the diamter at that point. I have a tubular manifold that I was going to install this summer with an 1-7/8" final output diameter.

2). Worn carb pistons not holding vacuum- I need another pair of HS6's or even a nice set of H4's to try swapping out in order to rule this out.

3). Valve train problem. I read an old archive post and the guy's symptoms seemed absolutely identical to mine. The problem was bent valves and uneven springs- possibly from sticking valve guides.

4). Just too much carb for the engine, is what I have been told by someone. Unless I can directly compare with some known good HS6s, I won't be able to determine that but my guess is people are running them on more mild engines without having the same problems.

I did a compression check- 200, 190, 185, 190 dry, and exactly 5lbs more on each cylinder on the wet test. Not sure what to make of #1 being 10 pounds higher, but the other three seem to be in line.

I'm not too well versed in terms of valves vs. manifold pressure, so I was hoping someone could jump in. Being a new head with bronze valve guides, I can definitely see the potential for a problem in the valve train i.e. sticking or floating valves, but I don't know how that would translate into lack of movement in the carb pistons.

Mark J Michalak


Although #1 might not be your problem, I recently did an easy fix by cutting the dent and using a connector with a clamp on each end. Dennis
Dennis Suski

My first guess-exhaust too restrictive. Second guess--gonna have to keep searching for the right needle profile. Third guess--too much carb for the engine.

What about ignition (and timing)? Electronic with the wrong advance profile--too aggressive at higher rpm? Is the temp rising abnormally at steady state higher rpm?
JM Morris

Additional comments: You've got to straighten the exhaust out first. Everything else will change accordingly.

JM Morris

Mark -
At the bottom is an edited version of my explanation of how SU carbs actually work, formerly posted in various forms.
Meantime, I will comment as you go there! Read everything over and over until you understand it.

" I've tried 3 different needle profiles: SY, RI, and BCA all with the same result- too rich at idle, too lean at speed."
>>Once you understand the basics, you will see that everything is based on piston position at load, which is directly controlled by the piston springs. SY is the factory needle for these carbs, but with "light blue" springs. There is NO reference in any literature including the BMC master SU list about "light" blue, so we have to take this as "blue". Blue springs are rated at 2 1/2 oz @ 2 5/8".

"I'm using the lighter weight red springs in the carbs."
>>Lighter than what? There is only one "red" spring, rated 4 1/2oz @ 2 5/8".
>>Without actually measuring everything. I can't say exactly what the installed change of load vs position is, but it is certainly true that the heavier springs you are using will result in the piston and needle being lower over the range, such that the top end of the needle never comes into play. Needle comparisons using the full normal operating length are then useless.

"Everyone including myself agrees this is not enough vacuum, but we can't figure out why"
>>That'd be because "everybody" doesn't understand how it works or how to measure it. This is an airflow measurement that must be taken at steady state wide open throttle - ie, on the road or rolling road. The airflow in the shop is only that required to make the unloaded engine go however fast it's going, a small fraction of what it is capable of. You can redline an engine on the idle screws, maybe 10% max of full open.

>>Yes, Until you fix the exhaust you are wasting your time. You could fit the HS6 with the correct springs and SY needles as a get by. I would refit the stock H/HS4 until the exhaust is sorted. When specifying exhaust sizes,state ID or OD. Preferably ID as that's what matters. You can take ID as 1/8" less than OD for practical purposes, assuming near 16ga tube. The ratios of cross section size by ID are: 1 1/2 (OE MGA):144; 1 5/8 (OE MGB):169; 1 3/4:196; 1 7/8:225; 2:256
Extracted from Vizard: Within this information, there are a few nods toward real life street use, namely, "a 1 7/8”OD pipe will give maximum power, but a 1 3/4OD one will give 1-2hp more from 4-5000rpm, while only costing the same amount from 6500 to 7500, on the same engine, a 95hp 1275." At what rpm do you propose to use your street car?

>>No, you already answered this - " the drop test is good" It's an understanding/ test problem.

>> Not likely, in view if the fact that you have made it work by richening the mixture AND we have an explanation.

>> Within possible limits (HS8 on Rover 2000TC), there is no such thing for SUs correctly setup. The Volvo mentioned and the factory comp arrangement plus countless other apps prove it. Anybody who says this with regard to SU simply does not know what they are talking about. I am well aware that there are a lot of people who should know better but don't.

The engine as described is little more than a decent early 1800, or more likely a 1600/1622 with a bit more low end. Top end would be a lot better, and different in regard to carb setup, with a decent exhaust system. The exhaust might allow the valve mods to work, or maybe not, depending on how correct they are. In any case, it would work very well with stock MGA H4, or MGB HS4 carbs, since they are not restricting power in this range. It should not be difficult to set up the HS6 to work, since you have them, but you are not gaining a thing.
A better cam would bring it to life, and then you would certainly need carb recalibration, but probably will never get to where you actually need the HS6 in street use - they only become useful when you exceed the power limitation of the H4, around 100hp actually developed, which means you are driving around at or above 6000rpm. There are some calibration advantages to be gained with HS6 near this point, in that they spread the load over a longer needle profile. Volvo B18 used HS6; pretty much the same size engine with a better head/manifolding, which is what you hope you have but will never quite attain.

Hope you find all this entertaining and informative!

Basic operation principles:

"Carb piston in a fully optimized setup reaches full lift at Max hp. Since you are hoping for more hp, you need a little leftover for improvement. In other words, if the power curve is still going up when the piston tops out, you need a stronger spring or more weight in the piston. Either will cause all lower settings to become more rich. If the power curve has started to drop before the piston tops, you still have room to go with the current spring/piston. If the power curve drops well before the piston tops, then you need a lighter spring, which will make all lower settings more lean. When you can't find a light enough spring, you need a smaller carb; and, if the spring is excessively heavy, you need or could benefit from, a bigger carb.

Power is produced by burning fuel with the correct amount of air. Assuming correct mixture, max air = max fuel = max hp. The piston does produce a restriction, but it is small; for a given carb it is controlled by the spring, and is effectively constant - that's why an SU is a "constant depression" carb, as are various other carbs with a similar vacuum operated piston. The depression is measured from one side of the carb to the other. The piston is nothing more than a "mass airflow sensor", as used on modern EFI cars. Instead of a lot of electronic gimmickery, the fuel is controlled directly by the tapered needle attached to the piston.
Inlet porting, valves, etc. cause changes in the vacuum transferred from the engine cylinder(s) to the output side of the carb; if these are improved the carb "sees" more vacuum and flows more air. CR increases and exhaust porting and other exhaust flow improvements actually increase the vacuum in the cylinders, again resulting in the carb "seeing" higher vacuum. The constant depression is used to raise the piston and metering needle to supply the needed fuel in the correct proportion. If the "constant depression" is too great, as would be caused by a too stiff spring, then the carb piston itself becomes a limiting restrictive factor, and since piston/needle travel is less, it becomes more difficult to keep the needle profile correct. The "too stiff" situation is used to advantage on quick throttle opening in the damper arrangement which makes the mixture temporarily richer, as required by accelerating engines. Flow improvements on the incoming side of the carb allow more air into the carb, but again do not affect the "constant depression" in the carb. Flow changes on the incoming side do affect fuel calibration, as any vacuum change here is applied directly to one end of the fuel column, the other end of which (at the jet) is in the "constant depression" area of the carb."
Fletcher R Millmore

By saying lighter weight red springs I simply meant that they are lighter weight than every spring other than blue. It's tricky too to say what the "stock" needle and springs were because it depends on which car, which application, etc. If you try to buy springs from Moss they want to sell you yellow springs for the MGC HS6 and green for the Healey HS6. I had already ruled out springs and needle combos since the symptoms persist with no damper springs and 3 needle profiles. I agree that there is no point in going further without addressing the exhaust so that will be my next step. Thanks for all the comments.
Mark J Michalak

Mark -
"since the symptoms persist with no damper springs and 3 needle profiles."
Are you saying that you ran it on the road with no springs? Or just in the shop with small piston travel?

My references to "stock" or "factory" springs/needles are from the competition tuning book, SY needles with "light blue" springs.

Fletcher R Millmore

I don't know if Mark found it informative and entertaining but I certainly did. I had a very similar problem with my 1275 midget (with one HIF carb, 10:1 comp, Kent cam, and free flowing header and muffler, electronic programmable ignition). Although it never motorboated, it did run leaner and hotter at higher rpms, losing power. I tried various needle combinations and was never able to solve it. A friend of mine finally suggested that I first hold the revs at about 4500 rpm (no load) then make adjustments to the jet to give highest possible rpm. It "worked". No more problems on the upper end and only slightly rich at idle. The only real way to solve a problem like this is to get it on a dyno.
JM Morris

Hi Mark. I notice you installed bronze valve guides. Did you ream them a bit oversize to provide adequate clearance when they get hot and expand? If you didnt, as the engine heats up, one or more valves (usually exhaust valves) can begin to "stick" in their guides. If this happens the engine will begin to lose power, and the exhaust note becomes uneven, since the engine is essentially running on less than four cylinders. I dont know if this is your problem, but I think it is within the realm of possibility, since it happened to me years ago. You might want to get the engine good and hot, till it runs rough, and then immediately pull all the plugs, and run a compression test. If compression is good on all cylinders when the engine is cool, but compression drops when it is hot, you have probably found the culprit. Running a compression test costs nothing but a bit of time, and at very least, could rule out one potential problem. Good luck and let us know what you find. Glenn
Glenn Hedrich

I did find it entertaining an informative. In fact I've been doing an awful lot of reading about SU's and their functions lately, the most interesting is learning to create your own needle profiles. Glenn, as I stated above I did do a compression test while the engine was at operating temperature. I posted the numbers dry and wet. I don't notice an increase in engine temnperature at high rpms- in fact it's been cool all weekend an it ran at 75mph for hours at 180 degrees. As long as I set the mixture rich I won't have any problems. I did consider the possibility of sticking valves due to lack of clearance in the guides, but my machinist recommeneded them and he's a well known and respected engine builder up here in the Chicago area. I know he builds race engines and I doubt he made a mistake in mine.

FR- I did run it on the road with no springs. I hit WOT in 2nd gear, and as the revs hit 4000+ it immediately lost power. Then I spaced out the dampers with some washers in case the damper was shooting up too far too fast and creating a lean condition, the washers would prevent it from going all the way up and create a rich condition on acceleration. Same result, so I put the red springs back in. So, yellow, red, no springs= no change. I know that a rolling road is a good diagnostic tool here, but it's not going to happen so I have to work with what I've got. Don't you agree that even with no load on the engine 1/16" of damper lift @ 1000rpm and 5/16" @ 4000rpm seems awfully low?
Mark J Michalak

You may want to check that your damper threaded screw has a hole in it to allow the air above it to escape to atmosphere. Otherwise the air above the damper will be compressed preventing it from moving up higher. Some SUs have this in the body of the carb so I don't know if this is applicable for yours. I attached a picture of one that had the hole soldered over to give you an idea of what to look for. Just a thought.

Ian R.

IJR Renshaw

Mark--I do hope you'll keep us posted on what you eventually find.

Re the damper lift vs the rpm in your last post: it would be hard to say without a comparable measurement using blue springs.

How and where did you attach the ruler for measuring? I'm going to do the same thing for my stock H4s, which I have just rebuilt and will be installing this weekend. Maybe we'll be able to come up with some baseline, ballpark figures.
JM Morris

Mark: I was reading the instructions for the SU tool kit and it was mentioned that the piston will be up about 1/16 inch at idle.

JM Morris

JM- I used a zip tie to attach a metal rule from a combo square to the stalk of the dashpot cover. Then I simply used one of the balancing flags from the SU toolkit against the ruler. My lift with the red springs was maybe 1/32" at 1000 rpm. The 1/16" was with no springs at all. Might be interesting to compare measurements but most likely they'd be different anyway since your carbs are smaller and the velocity would be different. Still, let me know what yours are.
Mark J Michalak

Mark-- Preliminary measurements show about 1/32 at 1,000 rpm. I assume you're using the B intake manifold. I'm guessing that our air velocities are about the same, and that the factory sized the carbs to achieve approximately the same air velocity, although of course different volumes are being passed relative to the carbs and cc's involved, to achieve basically the same constant depression.

So my guess is your vacuum is OK.

Assuming cam timing and ignition timing is spot on, I'm guessing it's still a needle profile problem.
JM Morris

This thread was discussed between 27/04/2010 and 11/05/2010

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