Whats Going Wrong

[wardy18 33 1 Cars]

Started the hillclimb 205 up yesterday for the first time since my last event in Alderney 3 weeks ago only to find it had wasnt sounding healthy AT ALL!!

 

It drove in the garage and parked up fine and all ive done since is played around the throttle on my carbs to make the throttle pedal a little lighter!!

 

Symptoms are:

 

- On start up will rev straight up to 3k

 

- Missfiring quit badly

 

- Revs drop back to normal idle when HT leads 3 and 4 (cambelt end) are pulled off, no change is either 3 and 4 are kept on, both have to be taken off for it to drop back down!!

 

- Once shut off, if you push the throttle open on carbs, gases/smoke are seen to come back out of carb inlets on the left carb, cylinders 3 and 4 again but nothing on the right hand carb!!

 

 

Very strange, maybe headgasket blown between cylinders 3 and 4?!

 

Last time i had similar issues with these 2 cylinders misfiring it was because the float in the carb was sticking closed and so not not filling the chamber with fuel but this is not the case this time

 

Ideas please................

 

I am yet to do a compression test as i have to borrow a guage but cant until tomorrow.

 

A little info which may help come to a conclusion, its a XU10 8v head on a XU9 8v Ally block (86mm ontop of 83mm)

 

Regards

 

Simon

[pablo 0]

money would be on the throttle cable adjustment. or one side being set wrong in relation to the other

[DrSarty 90 1 Cars]

money would be on the throttle cable adjustment. or one side being set wrong in relation to the other

 

I'd agree with Pablo, as logic says if it was fine before the last thing you touched, then the last thing you touched is most likely something to do with it.

 

Either that or it's pure coincidence.

[pablo 0]

you could disconnect the throttle cable, and uncouple the carbs and see what it idles like.

[wardy18 33 1 Cars]

I did think about this but the throttle cable obviously simply adjusts the rpm and wouldnt give it any reason to suddenly start missfiring!!

 

When i pull out the plugs from the offending cylinders they are very black and sooty whereas the other 2 are perfect

 

Also did a colourtune test and they are getting good spark!!

 

Just been out playing again, tried a few things but still the same problem

 

I will try what you said Pablo and completely disconnect the throttle cables to they can have no affect and see what happens

[Galifrey 1]

I did think about this but the throttle cable obviously simply adjusts the rpm and wouldnt give it any reason to suddenly start missfiring!!

 

When i pull out the plugs from the offending cylinders they are very black and sooty whereas the other 2 are perfect

 

Also did a colourtune test and they are getting good spark!!

 

Just been out playing again, tried a few things but still the same problem

 

I will try what you said Pablo and completely disconnect the throttle cables to they can have no affect and see what happens

 

Well sounds like it is overfueling on those cylinders, maybe coupled with some cable issues.

 

The smoke is usually evaporation of the fuel squirted by the acclerator pump, so maybe 1 set of carbs is open too far and overfueling for the rpm the engine is achieving. Partially stuck open on that set of carbs would explain the absence of the accelerator pump "smoke".

[wardy18 33 1 Cars]

Well sounds like it is overfueling on those cylinders, maybe coupled with some cable issues.

 

The smoke is usually evaporation of the fuel squirted by the acclerator pump, so maybe 1 set of carbs is open too far and overfueling for the rpm the engine is achieving. Partially stuck open on that set of carbs would explain the absence of the accelerator pump "smoke".

 

 

I do personally think it is Carb related, ie accelerator pumps. The carbs are balanced perfectly and both butterflys are completely closed so all i can think is a stuck accelerator pump somehow, i think my best best is pull off the offending carb and strip it down to see whats happening!!

 

This may have been caused by me constantly pumping the throttle when testing it as i was trying to lighten the throttle pedal so everytime i was doin this it would have been utilising the accelerator pump and squirting juice into the engine but not being burnt until i finally started it up satde!!

[Arahan 4]

Also check that the carbs are properly attatched, if theres an air leak from them it will drive and sound like crap.

[wardy18 33 1 Cars]

Also check that the carbs are properly attatched, if theres an air leak from them it will drive and sound like crap.

 

yeh this had popped into my mind but hadnt checked it yet, thanks for the reminder, its difficult with carbs as your not meant to have them fully tightened to save the fuel from frothing in the carb so i did loosen them slightly a month back!!

 

maybe with me pumping the throttle when testing the pedal might have puddled fuel up on the rubber O Ring connecting carb to mani and melted it away slightly!!

 

i have a couple of things to check tonight as i haven't yet got my carb balancer guage to check the suck on each cylinder, this may give me an insight if one carb is sucking harder!

[welshpug 1,657]

maybe its a sign!

 

 

 

 

[wardy18 33 1 Cars]

Get rid of Carbs and move to TB's......................

 

Im in the middle of a 2.3 Build so TB's will have to wait for a while!!

[taylorspug 7]

Its also quite suprising how many classes specifically allow carbs, but state that any fuel injection system must remain standard, or words to that effect. Which pretty much bans throttle bodies alot of the time. Its certainly worth checking, as i know my BMW cant go into a couple of classes no because of this. Not that im bothered, i can only do one race at a time!

[DrSarty 90 1 Cars]

I think it's often natural to assume a mis-fire is something ignition related; I used to.

 

But since experiencing air leaks due to injector hole bungs in throttle bodies dropping out (temporary ones I must add), I would say you have acquired one or more yourself.

 

These sudden gulps of air chucking the mixture out wildly are perhaps causing anything from a stutter and hesitation to almost a blow back (complete with bark or squeek) sometimes.

Edited October 12, 2009 by DrSarty

[wardy18 33 1 Cars]

I have much more to check after work, thanks guys, ill report back tomorrow!!

 

My money is on an air leak somewhere or accelerator pump!!

 

I ruled out ignition problems straight away after doing a couple of small checks

[Arahan 4]

Its always the way that the small and simpler things get overlooked for fear of a bigger problem!

[wardy18 33 1 Cars]

Fingers crossed, sounds much more logical than Head G or Valve issues!!

[wardy18 33 1 Cars]

ok here's the report of the latest investigations, carbs came off and everything looks fine, ie o ring seals, butterflys so its not sucking in any extra air from anywhere!

 

Started her up and put my carbs balance in the trumpets on the offending left carbs and its sucking off the scale whereas the other carb is sucking normally!

 

Wtf.........

 

Its as if something in the engine combustion chamber is causing extra suck on the carbs but even if I hold the throttle shut so the butterflys are held closed its still as bad so its like its actually sucking through the emulsion tubes/main jets/air correctors

 

I'm lost

[wardy18 33 1 Cars]

compression is fine, hitting approx 195psi in each so head gasket is fine and valves should be ok

 

I'm jus wondering if the inlet mani to head gasket has gone but you all may or may not have noticed in the xu9 8v head there is a little hole between/below the left 2 inlet ports the right 2 inlet ports, now because my inlet mani has corroded away slightly where it would sit over these holes I've had to use some chemical metal to seal the holes and build up the slight corroded recess in the inlet mani so that it all seals with the gasket! I have had the coolant wear through the gasket etc and leak into the engine before so maybe this has happened again

 

I don't know if anyone know the little 2 holes I'm referring to but its all I can come up with at the moment as I have honestly tried everything else

 

What peoples thoughts

[wardy18 33 1 Cars]

This is my XU10

 

Red circlied = POSSIBLE OFFENDING HOLES, these open into the coolant chambers in the head

 

Green lines = inlet mani stud holes

[GLPoomobile 958]

I think it's often natural to assume a mis-fire is something ignition related; I used to.

 

But since experiencing air leaks due to injector hole bungs in throttle bodies dropping out (temporary ones I must add), I would say you have acquired one or more yourself.

 

These sudden gulps of air chucking the mixture out wildly are perhaps causing anything from a stutter and hesitation to almost a blow back (complete with bark or squeek) sometimes.

 

Interesting (to me) that you've mentioned that. I'm reasonably certain I have quite a big air leak on mine because a} I've no inlet manifold bolt in the belt end, due to slightly stripping the thread, and b} I sheared the fuel rail bolt on the same side, so the rail is now only torqued down to the manifold by one bolt (both problems to hopefully be resolved soon by Mr Taylor, when I arrange a date with him). As a result of this, my idle is quite lumpy (if I unplug the ECU temp sensor, the revs rise slightly and the idle smoothes right out. This is because the additional fuel being injected must be balancing out the extra air from the leak). But the point I'm getting to, in a long winded way as usual, is that it's also quite unpleasant to drive at the moment, because there's virtually no light throttle adjustment (apply a fraction of throttle and nothing happens, apply a little more and it suddenly picks up), and it doesn't like anything below 2k rpm. It's very jerky and lumpy. Driving it around it does actually feel more like an ignition problem, and I did wonder if my coil and amp were seeing enough voltage as I recently had the problem with the loose switched live connection. I'm positive that these symptoms are just down to the air leak, but like you say, it feels like an ignition fault and certainly did have me wondering when I was driving it the other day.

[wardy18 33 1 Cars]

im pretty certain mine is something to do with JUST the left Carb or JUST left 2 intake ports on the inlet mani leaking because as soon as i take of BOTH HT leads on cylinders 3 and 4 relating to the left bank (cambelt end) the idle drops back to normal as its now just running on the good carb on cylinders 1 and 2!!

 

all i can think off now is the inlet mani to head gasket blown as my coolant has dropped quite a bit also and looks pretty muddy/merky!!

 

really need help as my event is satde!

[wardy18 33 1 Cars]

a twisted carb spindle.........damn it

 

Couple with the fact that I had backed off the carb throttle linkage and so lessening the pull back tension in the linkage return springs resulting in the spindle not being twisted back when the throttle closed and so leaving one partially open

 

All this together making the left carb alone rev up and so causing a subtle like missfire along with the high rev

 

Kinda sorted now but I really need a new spindle in that carb for next year!

 

Thanx for all the help and advice guys, looking forward to my last hillclimb this satde now

[Galifrey 1]

a twisted carb spindle.........damn it

 

Couple with the fact that I had backed off the carb throttle linkage and so lessening the pull back tension in the linkage return springs resulting in the spindle not being twisted back when the throttle closed and so leaving one partially open

 

All this together making the left carb alone rev up and so causing a subtle like missfire along with the high rev

 

Kinda sorted now but I really need a new spindle in that carb for next year!

 

Thanx for all the help and advice guys, looking forward to my last hillclimb this satde now

 

So I was pretty much bang on then

 

[TT205 42]

So I was pretty much bang on then

 

 

 

Gold star for Mr P

[wardy18 33 1 Cars]

It always had to be carb related as that was the last thing touched since it was running fine but working out what was wrong was hard, couple with an overstreched spindle return spring and slackened off linkage i wasnt hapopy at all!!

 

still got some fine tuning to do as the idle is still high with the twisted spindle but at least its good to use for my last hillclimb satde