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Blowback.....could this be MJ related?.
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Post Blowback.....could this be MJ related?. 
Hi All,

OK, I'm at my wits end.....

I have aproblem with my car....it blows back AF mixture at 5500rpm (irrespective of load) through the front weber (i.e. both barrels, up to this point it runs perfectly....


Engine is CVH with MJ, but as the engine has only just run in I'm only just notcing this (noticed on a recent Rolling road).......

I've since tried replacing a follower, changing the valve springs, changing the oil (just in case the hydraulic followers were pumping), changing my VR sensor bracket to something stiffer, etc etc...but still my car is spitting back fuel air mixture at at 5500rpm (irrespective of load)....Just can't figure out what would cause this to happen out of both barrels of the front carb.....Also checked HT leads, plugs, timing, etc, replaced coil pack..........

Clutching at straws, but if anyone has any theories on this, they would be most welcome as I'm about to torch the car!.......

By checking the timing I reved the engine to the point it starts to blow back and the timing seems solid, but would I see see the timing light change at those revs if there was something wrong...

Please feel free to ask questions.....


Ta in advance.....Paul

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Hi Jenko,

Try running the EDIS on 'limp home' mode to see if having the MJLJ connected affects anything; that should be an indicator.

Rough running / intermittent firing around 4000-5000 always reminds me of a mechanical sensor mounting issue caused by harmonic resonance- but you say you've made the bracket very rigid. If this was indeed the source of the problem it would mean that the EDIS module is loosing sync with the trigger wheel. You should be able to observe this effect as missed ignition events when watching it with the timing light.

What else... have you tried loading different advance values around that region to see what might happen?

Brent

(So after we get you sorted I have a sick turbo'd 4AG motor Smile - it's making loud ticking sounds from what appears to be the valvetrain - I'm hoping it's not the bottom end!)

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Hiya Brent,


I can confirm that MJ is working.....I've used a timing light to watch the mark on the toothed wheel, and it all works perfectly, I was just wondering if it was possible that MJ was doing something specific at this rev point.....

As for the bracket...I went through this when first installing, and indeed did have some issues with viberation....however, I've totally reworked the bracket and it still does exactly the same thing....

Gone from this:

http://i34.photobucket.com/albums/d102/jenkosrugby/DSCF1113.jpg

to this:

http://i34.photobucket.com/albums/d102/jenkosrugby/DSCF1447.jpg

This is rock solid, I cannot physically move it with my hand - I also put on a biger trigger wheel....but this has not changed the effect at all, so I really don't think it's the VR sensor.

As for the advance values...the problem happens when the engine is under no load (i.e. just revving it without being in gear), and also under full load (i.e. under hard acceleration)....both of which have different advance values at the point at which blow back occurs......

I don't think it's engine as it runs perfectly up to that point (i.e. any compression issues would show up sooner)...could still be valve train related (but again Ive checked almost everything here), it just feels like at that specific rev the fron two cylinders are loosing ignition.......but why would this happen?...if it was cylinders 1-4 or 2-3 I may think it was coil pack related....but it's 1 and 2.....just throwing ideas around, and of course it may mot be MJ related, but the point at which blow backoccurs is so consistant it got me thinking MJ could be a reason......


Will happily throw ideas around re you sick 4Age (that engine would sit very nicely in my Westfield Smile

Jenko.

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Shocked that is a rigid bracket!

My question about disconnecting the MJLJ is that it removes it from the equation- with the MJLJ disconnected the EDIS module will revert to 'limp home mode', fixed at 10 degrees BTDC. Do you still see the problem then?


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Post pressure waves!! 
just a thought

u can get blow back jsut at certain revs because of the pulsing effect of air getting drawn into a carb, it wud take me a long while to describe it so look a little into pressure waves and induction length tuning. this could be the problem! failing that it could be a fuelling problem, engine running lean!


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DO SOMETHING SILLY, TURBO AN 1100CC METRO Smile
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I'm not sure running in limp home mode (with a 10deg advance), would be much good for the engine (as it needs to be revved to 5500rpm to see the problem)...infact, I'd be surprised if it even got to 5500rpm...Could be wrong though......

As for pulsing effects....how would I go about proving it / checking it?.....


Thanks so far for all advice.....

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Been thinking about this since you posted - and had the same thoughts as you before you posted about it being odd if it is ignition related as it's on 1&2 whereas you might expect it to be on 1&3 or 2&4.

So the common thing is the front carb - are you sure it's identically choked and jetted, float levels (min and max) OK (and the same as the other carb etc etc. Have you taken all the jets out and checked for foreign objects etc etc?

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Hiya Martin,

Alas, yep sure the carb is the same, it was origionally set up on a rolling road after winter rebuild, where both carbs were re jetted to achieve the correct AFM......

I tried swapping floats to see if that made any difference...it didn't....must admit, it does sond like the only common component is the weber......but apart from floats, and needle jet, I'm scratching my head trying to think what else is common (There are two of all the jets)......

I think I need to swap carbs and see if that makes any difference............?.

Cheers.

Paul.

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Unlikely to be ignition as only the front carb is involved.
So, let's try to clarify what you have and what you have tried.
Carbs mounted with soft mount kits including the correct multi turn spring washers with Nyloc type nuts?
Any vacuum sources being taken off the manifold and if so does it affect only one barrel, one carb or everything?
What length inlet trumpets on the carbs?
Float levels - have you tried adjusting the level on the front carb a little at a time, first up and then down?
Can you set up a temporary support (even with stiff steel wire) from the carbs to a solid carrier from the top of the motor?

You may find that the front carb suffers from a different harmonic vibration relative the rear.

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Hiya,

OK..to answer you questions....

Carbs mounted with Misab mounts, and using correct wasers, and rubber spacers (set to 1.5mm gap)......

No Vacume sources taken from carbs.

Inlet trumpets are weber 26mm.

Float levels have been checked and seem fine....I even swapped the carbs over this weekend to see if the fault followed the carb....Alas, It didn't, so unless there is an issue with the inlet manifold, I can't think of anything else that would effect the front two cylinders (i.e. that is common to the front two cylinders)........

Also tried playing around with Vernier timing but no difference......Is there any point trying to adjust the toothed trigger wheel slightly.....does this change timing?...............


Thanks.

Paul.

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valve clearances/hydraulic lifters? valve bounce (weak valve spring)? doesn't sound ignition related as stated, you'd expect to experience from both carbs. A cylinder leakage test should show up a badly seated valve. Have you swapped carbs "front to back" to see if the problem follows the carb?

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<<Have you swapped carbs "front to back" to see if the problem follows the carb?>> errr....he said he has - and it doesn't follow the carb Smile

Yes, you can adjust the trigger wheel to change the timing, but apart from it affecting the 10BTDC EDIS limp home mode it doesn't do anything (within reason) that you can't do with the programmable ignition map.

You've got the MJLJ rev limit well above 5500 rpm haven't you?

I suppose you could try swapping EDIS pins 10 and 12 over and rewiring the plug leads so that 1&3 and 2&4 are using the other coil to the one they are currently using. Wouldn't pin too many hopes on that though....

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Umm, the problem didn't follow the carb so it's either both 1&2 related or both carbs are marginal and the front carb location just happens to be more problematic.
A cylinder cranking pressure ('compression') test might reveal a problem common to both cylinders. A head gasket blown between cylinders could do it with your firing order.

I'm inclined toward the mixtures leaning out but the parameters in your MJLJ might be at fault too.
Raising the float levels a little may give an indication. So may a set of longer inlet trumpets. Good quality filter socks are better than open trumpets unless you can control the airflow around the inlets.
Don't assume that you must have the same float level in both carbs - a good number of multi carb factory engines use different settings.
In any case a good exhaust gas analyser or wide band oxygen sensor will show what those mixtures are really doing.

Even though this should affect both carbs, I assume the settings in your MJLJ continue well past 5500? ie there is a sensible advance figure at 6000 or more? The rev limiter is set well above 5500?

I'm surprised that the rolling road operator wasn't able to give you a good idea of what was happening.
He/she should have been able to see mixture indicators & power fluctuations at that point and experience would have told him/her what the likely source was in light of the spitting back on the front carb only.
I think I would be looking for another tuning shop, preferably with a good record with known competitive race cars.

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sorry... missed the bit about swapping carbs...

A mate of mine had problems with a DCOE spitting back and that was caused by an air leak weakening the mixture. I find it almost impossible to beleive that it could be the ignition affecting both 1 and 2, but not 3 and 4.

Have you taken the plugs out to look at the colour of them / what deposits there may be... To me, it sounds like a weak mixture on that carb.

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Air leak is a possible cause of lean mixture although affecting both barrels of a Weber is a bit unusual.
Test is easy - use a can of WD40 or similar water dispersant with a thin tube nozzle.
Run the engine at 2000 or a bit more, squirt around manifold/head gasket, carb mounts, any blanked off fittings on manifold etc.
You should get rough running and/or white exhaust smoke when you hit the right spot.
Actually any light oil in a squirt can will suffice, just harder to clean up.

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