82 replies to this topic

[bigmatt4]

inside on mods done to engine....
i have just built a new 1340 mini engine all parts new bar crank rods block and head 10.2:1 hepolite pistons 12g940 head standard valves (new) double springs hardend seats fully balanced bottom end crank flywheel balancer pistons rods running a 649 cam moderate headwork removed 70% of the guide bosses. port sizes to vizard specs and reworked behind the valves to help flow fitted with a 40 dcoe weber 35mm chokes.
now the story begins ____ first start of engine stared first compression ran to 2500rpm to run in cam engine ran smooth temps oil good as ran for 15mins shut down retorqued head valve clearances set to .016" as cam specs inspect plugs nice biscut brown colour restart engine high idle still test drive goes like a rocket little flatspot not to bad done few ks ok time to tune drop idle set idle mixture screws idle verysmooth not lumpy at all exhaust sounds strange like not idiling on 4 few tests remove spark lead no 1 no change no2 big drop in idle almost stalls no 3same as no2 almost stalls no 4 no change strange rev up with no leads on 1 and 4 very flat missing put leads back on revs real nice and clean small flatspot still. strange not running on 1 and 4 at idle remove plugs 1 wet and clean 2 dry and brown 3 dry and brown 4 dry and light brown .compression test 180psi all cylinders very strange saimese port cyl 1 and 2 share also 3 and 4 one cylinder per port not running . ok next test start engine with just 1 and 4 ht leads on engine starts wont idle low runs rough but on both cylinders checked manifold and carb for vacume leaks no leaks. very strange suspect cam timing remove timing cover check double check triple check all good no1 intake valve fully open at 103 deg as specs say . rang cam guy says big cam no vacume idle be crap. ratshit street car no good remove engine fit new sp13 cam 276 equv cam fast road less duration sweet should solve idle problem timed cam to 106 deg as specs say start engine run in cam 15 mins drop to idle stil same what this cant be drove car nice power lots of pull from low revs nice still idle fault still flatspot solve flatspot increase idle jets from 45 to 50 flatspot almost non existant do hot shutdown medium light throttle remove all plugs all even nice brown colour little rich fine for now so that eliminates main jets idle engine for 2 mins or so remove all plugs 2 and 3 little rich not fouled rich no1 wet no4 dry still no idle 1 and 4. next test remove 2 and 3 spark plugs and start engine on 1 and four adjust to and low idle runs nice ok refit no 2 and 3 spark plus with no other adjustments sprk leads still off start engine wont idle running rough shutdown refit spark leads 2 and 3 and removed 1 and 4 plugs started engine ran nice low idle with no adjustments . what the hell ok spoke to a few people none have ever heard of this hapening even tried restricting the exhaust for more back pressure if i put 55 idle jets in and richen mixture up it almost runs on 4 cylenders at idle but fouls the plugs im starting to run out of options i have tried 2 different 40 dcoe webers that are in perfect order and also a 45 dhla delorto with adjustable air bleeds still same symptom also put back in a standard points distributor to eliminate dizzy fault . i am thinking towards a scavenging fault and the 2 centre cylinders are robbing the good a/f mixture and two outside getting the crap left over but why and anyway to overcome this fault . i have had alot of experence porting heads on cortina escort and triumph engines with great success and have two cars with twin webers and delortos self tuned and they run great but this mini has got me stuffed i am lost anyone help me

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[Yoda]

Dizzy looks to be in a strange position to me! can we get a close up of the dizzy and HT leads from the front? usually the Vac advance is facing toward the thermostat / front of the engine (fan end) but i know the engine can run with the dizzy timed in more or less any position as long as the firing order is correct etc, but i believe it is best to set it up correctly to make sure the ignition is spot on (factory) before looking at carb issues. Maybe if the timing is incorrect then scavenging could be an issue as you have already stated.

I am willing to be corrected, but i cant think of anything else from experience that would cause this. Obviously you have tried several carbs without any change in symptoms, so i suggest looking elsewhere first.

If the dizzy drive spindle is not set correctly then the timing could be out enough to cause your problem, particularly with a hot cam!

Edited by Yoda, 22 August 2011 - 09:31 AM.

[Gulfclubby]

It'd be so much easier to read your text and try to answer your questions if you made even minimal use of punctuation. Jeez...

Edited by Gulfclubby, 22 August 2011 - 09:51 AM.

[minimender]

What is your advance at idle?

Should be between 10 and 12 degrees depending on dizzy.

If you don't have enough advance at idle then it will run lumpy and may not be burning the fuel from that Weber

Some highly tuned engines need 15 deg or more at idle to run properly.

Your inlet / exhaust port velocity (thats a clever word isn't it) will be slow at idle and the engine wont be happy until the revs rise a bit.
More advance at idle will smooth things out but you may not have the right dizzy in to be able to do that.

[Cooperman]

1. What plugs are you using?
2. Have you tried changing the plug leads.
3. With that cam and carb you need the tickover to be around 1100 rpm. AS 'minimender' says, the inlet charge velocity is critical on an engine like yours.
4. Logically it has to be an ignition fault as you've changed cam and carb(s). Is the distributor cap OK?

Ideally a rolling road session should identify the problem.
Are you sure it's 1340 cc. That's a strange size as +0.060" gives 1330 cc.

Edited by Cooperman, 22 August 2011 - 10:22 AM.

[bigmatt4]

sorry mate yes it is 1330 i have re curved the dizzy. at idle it setting timing to about 12 to 14 deg. maximum advance of 36 deg at 3000rpm dizzy is in wrong place yes but i have done that for the looks have tried a standard 1275 points dizzy with different leads and have swapped plugs between the cylinders have had engine running while advancing the dizzy to 25 deg at idle still no pickup of cylinders tried tuning no 1 and 4 cyl on ther own timing mixtures and refit leads 2 and three and it brings idle to around 3000 rpm it picks up the other 2 cylinders at about 1700-2000rpm transition before picking up the cylinders is very smooth you would swear standing at the engine it was idling on all four you could almost balance a coin on the engine at idle not quite but you no. also it has no cammy idle but wen you drive it at about 2500rpm at cruse you can hear the cam i am using ngk bpr6es plugs note: in the picture is the points dizzy i have the electronic dizzy fitted used point dizzy to eliminate dizzy fault because they are basic.

Edited by bigmatt4, 22 August 2011 - 08:15 PM.

[Cooperman]

This is all very strange. I am finding it a bit hard to read the posts due to lack of punctuation, but think I understand the problem.
In essence then, when at low revs it is only running properly on cylinders 2 & 3 but it's fine at over about 2000 rpm.
As the head is siamesed on 1 & 2 and on 3 & 4, it can't be a lack of fuel-air mixture getting to cyls 1 & 4. Also the mixture inlet velocity will be the same on 1/2 and 3/4. The only possible exception to this would be if the porting/gas-flowing were non-symetrical or if the compression ratios were badly inconsistent and this seems unlikely.
So, if the correct, or at any rate the same, mixture is reaching all cylinders in the same volume/cylinder, what is left?
I'm assuming it's not a 'scatter pattern' cam, as these have been known to give some strange running low-down at critical settings.
The only other issue is the spark which can be either weak on 1 & 4 and/or happening at the wrong time. But the distributor has been changed with no improvement and the timing has been checked and tried at different settings.
We are all missing something here, but it's hard to see what. Normally when 2 cylinders are not fully functioning it's 1 & 2 or 3 & 4, which makes diagnosis straightforward. When it's not the siamesed port cylinders, as here, it usually turns out to be plugs, leads, or a defective dizzy cap. I once had a cap with a hairline crack which caused this sort of fault but one assumes that you've tried a different cap already.
Sorry I can't be more definitive, but we are all struggling to define the problem here.

[Yoda]

I was wondering myself if it could be down to a scatter cam and slightly out timing giving fire on the cam overlap but there is no mention of backfiring or tinkling in the exhaust manifold. Agreed two cylinder issues is usually 3/4 1/2 and the only other twin misfiring cylinders i can think of is blown head gasket between 2/3 which would not be right in this case. I seriously doubt it is carby issues due to the same reasons as Cooperman states. and again, i still think it must be an ignition timing issue. but again, am i missing something obvious.

even the cat cant think of the answer!



Thinking caps on again i suppose.

Edited by Yoda, 23 August 2011 - 10:30 AM.

[Cooperman]

Maybe it would be an idea to try a different plug. How about a BP7ES for starters or even a different make like Bosche or Champion N9Y.
Then fit a different distributor cap and rotor arm. This really is 'clutching at straws', but it must be something unexpected here.
Could plug leads 1 & 4 be 'tracking out' somewhere? Run it in pitch black conditions and look for spark-tracking.
A really radical idea would be to remove the rockers and push rods for cylinders 2 & 3 and run it as a 2-cylinder engine with no mixture going into 2 or 3. This would show whether 1 & 4 were actually able to work effectively. Since 1 & 4 are opposite, the engine would simply be like any 2-cylinder unit, but the inlet gas velocity would only be 50% of nominal so that would give a somewhat false impression. At least it would show the ability of 1 & 4 to work.
We need more ideas here.

[Yoda]

Am i thinking wrong here, but surely you would at least need to remove the plugs and ideally the exhaust valves for this test to be worth doing or you will be seeing compression and vacuum lock in the non working cylinders?

Reading back, i think i see mention of swapping plugs around but not actually the leads, unless the other dizzy had a different cap and leads, then that more or less rules those ideas out.

[Cooperman]

Am i thinking wrong here, but surely you would at least need to remove the plugs and ideally the exhaust valves for this test to be worth doing or you will be seeing compression and vacuum lock in the non working cylinders?

Reading back, i think i see mention of swapping plugs around but not actually the leads, unless the other dizzy had a different cap and leads, then that more or less rules those ideas out.

I was assuming that the plugs would be removed from 2 & 3 if running it as a 2-cyl engine. Thought that went without saying
This is very interesting and it will be great when it's sorted to know what the problem was.

[Yoda]

maybe there is more than one issue here, i just reread the OP and it seems cylinder one is getting wet plugs and cylinder 4 is staying dry. I am starting to pull my hair out now, i just wish i was there to see what is happening for myself. Still wondering about head gasket not sealing properly. having had the o ring shift on a head gasket preventing a good seal in the past, Maybe we should be looking along these lines. I know the compression readings were good, but Hey, we are still clutching at straws.

When we find the answer, i bet we all see how obvious it was all along!

[Yoda]

I was assuming that the plugs would be removed from 2 & 3 if running it as a 2-cyl engine. Thought that went without saying
This is very interesting and it will be great when it's sorted to know what the problem was.

My mum told me never to assume anything in life!

Come on liirge, jump in here, i know you are snooping

Edited by Yoda, 23 August 2011 - 02:06 PM.

[Cooperman]

Plugs normally get wet when the mixture is going in and the plug is not firing despite it having compression.
A dry plug on an under-performing cylinder normally indicates low compression or a poor sparc or a 'late' spark (timing-wise, that is).
I wish this car was 'just around the corner', as I would love to get it in my workshop and have a go at it. A good challenge indeed.

[Yoda]

Looks like we should move nearer to each other and get the car shipped over because i think its in new zealand.

I bet we could get it sorted with our two Hammers (large) working in tune with each other

You just know its going to be something really stupid though!