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MG MGB Technical - Idle Setting With Kent 286deg Camshaft

Hi MG-Maniacs,

finally my engine revision is completed. And the troubles continue. Does anybody use a 286 degree Kent cam with standard cyl.head and dual HS4-carbs with K&N-Filters on an 18V-engine (or similiar setup)? How did you set up idle ? I am not able to achieve a steady setting below 1500 revs. I suppose, watching my colortune spark plugs, that the mixture is too rich below 1500 revs. However the adjusting range has expired. I will probably have to change needles from
Number 6 to a leaner type again (thinking of my old FX-Needles).
I would appreciate any comments on valve clearings, ignition timing (i use a 25d4-dizzy) and needle recommendations or sources for more info on MY camshaft. BTW the idle itself is fairly smooth, besides that it drops to nothing when set up below 1500 revs.
Does 286 degrees of valve seating as stated for this Kent cam mean that the contact time of the valves with the seats/cyl.head is greater or less than that time with a standard 244 deg. smog-camshaft ? I am a bit confused about this topic...

Thank You for your ideas

Mark


BTW.: I am about to check this setup with dual O2-sensors in outlet manifold soon...
Mark

The 286 cam refers to the amount in crankshaft degrees that the valves are effectively open. Thus when compared to the 244 the valves are off the seat and open for far longer. I have never been able to set up the idle of a 286 cammed engine much below 1200 RPM. Regarding the ignition timing the distributor is probably not the best one to use in standard form, it also depends upon the C/R of your engine and octane rating of fuel used. The best solution would be to have a rolling road set up, failing that if you were to set the timing for 33degrees full in at 5KRPM and the engine did not pink under low rev hard acceleration, then it would probably be pretty close.
Bob

Bob,

thank you for sharing your experiences. My Engine is a 18V883L , i.e. low-compression. Pistons are oversized by only 0,020 Inch. I have read elsewhere on the web that recommended valve clearance for the Kent 715 (i.e.286deg.) cam is 0,016 Inch. Is that right and does it refer to both inlet and outlet valves ? My Engine really does pink very noticeably under low-rev-acceleration meaning below 3000 RPM. This means ignition should be set more retarded, doesn't it? Engine runs on Euro-Super 95. Do you think 98 would do generally better ? As the engine is not yet run in i hesitate to rev it up to more than 3500 RPM, but does 33deg at 5KRPM include both vacuum and centrifugal advance or would it be recommended to disable either advance mechanism once at all ? Looking up approximate data for my 25D4-Dizzy numbered 41485 i read about 5 degrees max.vacuum advance and some 40 degrees centrifugal advance at 5KRPM adding to 45 degrees max. advance. Provided this i should retard ignition timing from actual/recent 15 BTDC at 1500 RPM to at least 5 at 1500. Could this work as a rule of thumb until i will be able do rev the engine up to 5000 ?

Have a nice weekend

Mark
Mark

Mark, Vacuum advance is a little strange and I personally would disconnect it, then if your figures are correct (40 deg cent.) This would set the static at about 7 degrees ATDC (40-33). This would then mean the car would be a piece of crap at idle, connecting the vacuum at this point could bring it to 2 degrees but the consequencies at 5K would be unclear. A better solution for you would be to keep retarding the ignition until you can clear the pinking at hard acceleration, at which point no damage would then be caused to the engine. The use of 98 fuel would greatly assist, allowing more advance than 95 before pinking was encountered. The car needs a distributor with less advance to run well. I use a gap of 15thou (0.015inch) with these cams on both inlet and exhaust.
Bob

Mark

Have you put new jets in your carbs?

If not it is likely that they are worn, particularly if you are on swing needle rather than fixed needles

with unleaded fuel the jets now tend to be very worn after 20,000 miles, and it makes it impossible to set the carbs up properly

this would account for very overrich mixture at low rpm/throttle openings, concealed by higher airflow, and less wear , when less of the needle is in the jet i.e. when the piston is raised under normal running

also you should note that the specification of the vacuum capsule is not governed by the distributor number - if you clean it you will find numbers, often 5-13-10, on the slimmer end near the vacuum connection, that is , outboard of the capsule itself

this means vacuum starts at 13" HG depression, finishes at 5" HG depression (as the throttle is opened) and provides 10 degrees of distributor advance, that is 20degrees crankshaft advance

I would not disconnect it. It does not affect full throttle running, but may cause part throttle pinking if the rest of the set up is overadvances

set it up for no more than 14degs static advance, then check by strobe that with vacuum disconnected there is no more than 34 degs advance, then reconnect vacuum

if still pinking it is more likely that you have weak mixture under load, probably caused by mistakenly setting too weak because the idle is artificially rich cuased by worn jets
chris

I've had a Kent 715 (286 deg) in mine for about seven years. I've always had the best luck with 1000-1100 rpm idle. I would make sure the timong is spot on and take a good look at the carbs.
Richard Smith

Chris, thank you for your ideas. I will go and have a look for the advance mechanisms numbers, soon. My carbs were all freshly revised some months ago. But indeed I have come to the conlusion that i am running to lean, looking at the spark plugs after trying to reset the ignition timing in 'idle'. The next thing i have to do is to adjust the valve rockers clearances to correct i.e. 0,016Inch values. I have got this number from Peter Burgess. My big problem in doing this is to rev the engine by hand. Before that revision it was rather hard to be done by the pulley. Now, even when the engine is still hot it is absolutely impossible to rev it there. Maybe i should try it with 4th gear engaged by rotating the drive shaft from underneath the car. How do you rotate your engines?
Richard, i am happy to hear any bid lower than 1500 RPM at which the idle could be set with that Kent715 camshaft, thanx. Was your cyl head modified or not ?

Mark
Mark

My cylinder head has been only slightly modified with some opening and polishing of the ports.

I am running a Mikuni PHH44 sidedraft carb and probably need to finish the head I have sitting on my work bench...

But, I think that if you get the timing sorted out, you will be able to get down to that idle. If you are running SU's, make sure you don't have worn throttle shafts. This can make a bear to get the idle down. I ran for several years with worn shafts and had to set my idle on 11-1200 simply because that was the only way I could get them to balance at idle.
Richard Smith

Mark


Have you tried a very large socket on the pulley nut - this usually will work, and you can reach it from underneath the car?

To turn engines over I always put it in 4th gear and push the car - holding a front wheel and rotating it in the desired direction will give you a lot of leverage

If you have new jets but are still very overrich, have you made sure that the choke is not operating the jets? try disconnecting the cable , and making sure that the jets come hard up against the stops

also try screwing the jets right up with the dashpots off so you can see what you're doing, then screw them down 12 flats from the point where the moveable part of the jet is level with the fixed surface- this should give you a slightly rich point to start from, and gradual weakening should bring you to a reasonable mixture
chris

Chris,

yesterday i succeeded in turning over my newly rebuilt engine using an adjustable 12inch-wrench i bought. This was fairly easy to do from above...
My new problem is that even a new 12v-battery gets soaked empty by night. I am sure the guys in the workshop have connected sth. wrong with the alternator/starter. It really is annoying.

Mark
Mark Leitiger

Mark

Tracing current drain sometimes can be done by disconnecting the live batter lead and connecting an ammeter between it and the live terminal

If there is drain this will show on the ammeter

Then pull each fuse in sequence - if it makes no difference, the drain is not from that fuse circuit - but if the drain stops on the ammeter, then you have identified the circuit with the drain of current

You then need to look for what it is eg lights staying on et cetera

If no drain shows up then it is an unfused drain of current - more difficult to trace but try

overdrive solenoid
coil
fuel pump
radio

Chris
chris

I definitely agree with Richard -- set your idle around 1000-1100. Low-speed idling absolutely eats performance cams. I mean that literally -- the lifters can make quick work of it. Faster idling is just SO much less abusive to them. I learned this a long time ago; recently I'd decided it was an old wives' tale and guess what, my new cam lasted all of about 15K miles and then had to be replaced due to a flat lobe. There's a very good reason why, when you buy a new cam, it says in the instructions DO NOT IDLE until you've broken the cam in. It's just murder on 'em when you have a more aggressive lobe profile than stock.
Ted

Ted, very interesting information you are pointing out. That gives me a little too think about.

Chris, however I have isolated that fault with draining all current from my battery overnight. It seems to be related with the alternator. If I do not disconnect the 3 wires I encounter much more than 3 Amperes being drawn from the battery... (My Meter stops at 3 Amps). All this even happens when all Fuses and ignition low-tension wires are disconnected. Can it be supposed that not the alternator but a wrongly connected starter or -solenoid is the cause ?

Mark
Mark Leitiger

Mark

Im not an expert on alternators but it sounds to me as tho the control unit is faulty, allowing the battery to pass current thro it - I imagine diode faults etc could allow this

I should take the alternator off the car and take it to an auto electrical repairer who will be able to test it for you

Chris
chris

The solution for this electrical bug has been very simple. My workshop just has connected the 3 wires-plug rotated by 180 degrees, which makes the car run and start properly, but on the other hand drains the battery. There seems no mechanical barrier to prevent this, at least on my alternator...

Mark
Mark Leitiger

This thread was discussed between 16/08/2002 and 26/08/2002

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