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302 Clevo Backfiring...Help 

 

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 Post subject: 302 Clevo Backfiring...Help
Posted: Mon Nov 14, 2005 10:17 am 
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Hi All, I'm just about to start tearing my hair out with this problem I have with my XC 302 Clevo
I goes somethin' like this.......

A couple of weeks ago I started getting some real bad tappet noise (I mean REAL BAD) anyhow I ripped the inlet manifold & rocker covers off to find that 1 rocker had about 5mm of slack (can't believe the push rod didn't come out) so I pulled the lifters out to find a cam lobe was allmost gone.
So out came the cam and replaced it with crow cam Part No 21771, I also put in new lifters & high performance Crow valve springs, I also made sure the cam was on Dot to Dot as the preveous cam was advanced on the rollmaster.

Now the problem begins...
Since then I am getting lots of backfire through the carbi and on the occasion through the exhaust, On 1 occasion I got a pop through the carbi & exhaust AT THE SAME TIME????

I've adjusted the timing countless times which is an OPTOSPARK Electronic dizzy and nothin' seems to get better.
The bit I don't understand is that before I changed the cam the car use to FLY, now I'm flat out gettin' it moving.

PLEASE HELP BEFORE I REACH FOR THE "WILL FIX ANYTHING TOOLBOX" WHICH ONLY CONTAINS 5ltrs PETROL AND A MATCH.

Thanks
Squirt :D

 

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Posted: Mon Nov 14, 2005 6:30 pm 
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Have you had the rocker covers off. This has happened to me forgot to tension down a rocker and it let the pushrod jump out :oops: . Other than that pull off the inlet manifold and have another look and dial in the cam whil your there.

hope this helps.
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Posted: Mon Nov 14, 2005 6:46 pm 
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It wouldn't be the first time that dot marks on cams weren't exactly accurate thats why it's a good idea to use a multi keyed (9) sproket and a degree wheel and get the exact lift at the exact degrees before TDC. I feel that for what ever reason your cam timing is out. i hope I am wrong but if not it's time to get your hands dirty again with a dial guage and degree wheel.
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Posted: Mon Nov 14, 2005 7:26 pm 
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Well you say you lined up the cam right so that shouldn't be the problem (even tho it sounds like it).

The other thing is check the compression on all cyl's and if they're all over the place then maybe the valve's aren't closing fully with the new gear???

Only a guess but somewhere to start. :D :D

 

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Posted: Mon Nov 14, 2005 7:53 pm 
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Thanks for all of your replys.

I've done a bit more tweek'n today I have found that if I rev it from idle HARD I get backfire, If I increase the revs slowly or medium I get no backfire.

To me this must be timing related (ignition), almost like the vaccum when I dump it advances to the point of backfire but when I'm a little more sedate with the throttle the vaccum isn't as great so it doesn't happen?
What i tried today was to retard the dizzy and bump the idle up and see how for I could go and have it still run. Well I test drove it like that & without a word of a lie I got pumped by a kid on a pushy. :evil:

Honestly this has got to a point now where I do not know where to look next.

Quote:
Have you had the rocker covers off. This has happened to me forgot to tension down a rocker and it let the pushrod jump out

Yes I even tightend them with a torque wrench

Quote:
It wouldn't be the first time that dot marks on cams weren't exactly accurate


It has a rollmaster in it, I know that doesn't guarentee it's right but it was fine with the other cam

Quote:
maybe the valve's aren't closing fully with the new gear???


This is the only other conclusion I have had but as I said it only does it under heavy load.

:?:

 

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Posted: Tue Nov 15, 2005 11:20 pm 
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BANG sparkplug fires at 6deg before top dead centre,the piston is forced down the bore the exhaust valve starts to open lets say 30degree before bottom dead centre, at 180 degrees the piston is at bottom dead centre,then the piston starts to push all the exhaust gases out through the exhaust valve ,when the piston reaches topdeadcentre it would have most of the gas out so the exhaust valve should be fully closed at top dead centre OR 5DEGREE AFTER TOP DEAD CENTRE ,I DONT KNOW WHAT DURATION YOU HAVE ?solid lifter or hydrualic, turn the crank back and forward around top dead centre if you have solid lifters get the feeler gauges out, turn the crank clockwise until you can get0.010 valvelash,if the timing mark on the balancer is more than 15 degreeATDCyou could be in trouble,i might cop a lot sh.t over the cam timing but it might just help you before you blow more money.. ignition timing should be 6deg btdc give or take 1degree number 1 sparkplug is onthe driverside front of block ,if thats right then check the harmonic balancer to make sure the timing mark is correct with number 1 piston at full top dead center ,,if that doesnt help you might have to speak to Phil at kelford cams his workshop is on lawrence drive NERANG if you dont want to go to him then theres a bunch more performance shops in that industrial estate. p.s,dont go to MARKS AUTO MOTIVE in jayjee crt, Nerang he likes doin burnouts hence his nickname[ skidmark] I dont no how much you know about cam timing but if you could put the cam specs up on the page we might be able to help a bit more, i once made a home made degree wheel out of a 5litre paint tin lid and worked out that my cam timing was advanced by 1 tooth too many but that was a mazda 808 wgn ohc1300cc ,once i retarded the cam timing by 1 tooth i could chirp 2nd gear not bad eh.manylauphs, buy the way i got the cam from kelford kams and i remember now that he has some cheap degree wheels down there.if you can find out where RUSSELL JONES performance went to he could help in a big way he knew his sh.t around clevo"s and i think he did all the machine work on Jim reeds top fueler as well.His old workshop was in Labrador ,if anyone knows were he moved to please leave a message as ive been overseas for 4years

 

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Last edited by 68keith on Wed Nov 16, 2005 1:23 am, edited 1 time in total.

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Posted: Tue Nov 15, 2005 11:35 pm 
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Could be lean condition also? Is your accellerator pump working o/k and NO inlet leaks?
Ignition leads on right?
Float level etc...

 

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Posted: Wed Nov 16, 2005 9:09 am 
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Ok here is the cam specs

Hydraulic lifters
Crow cam #21771
Valve Timing
inlet: 18/60
Exhaust: 62/20

Valve Lift
Inlet: .460"
Exhaust: .470"

Crow Performance Valve Springs
Twin springs #7738
Free length: 2.155"
Spring Rate: 350
Solid Height: 1.140"

I know it doesn't mean it's right but it also has a new H/Balancer instelled
The only conclusion I can come up with is the cam is retarded, BUT, if it was that retarded you would think that it wouldn't run properly at all.

I also had a thought that it might be riding valves, but with twin valve springs you would assume that the new lifters would bleed down with that amount of spring pressure. :?:

Here is a list of what was done:
Replaced,
Cam
Lifters
valve Springs
Harmonic Balancer
Inlet Manifold Gasket (with proper TIN one not paper gasket)

Other work done:
cleaned all faces
valve reseat (This was done will grinding paste and a cordless drill, I know all valves vent back into original positions. This was more of a tickle than a reseat)

Thanks again for all of your HELP :D :D

 

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Posted: Wed Nov 16, 2005 7:34 pm 
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CHECK THE DISTRIBUTOR ROTOR TO DISTRIBUTOR CAP SPARK DISTANCE TO A NEW CAP AND ROTOR i remember my little 808 wgn gave me hell once and thats all the problem was!

 

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Posted: Wed Nov 16, 2005 7:50 pm 
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you might be able to get one of the blokes at cheapa auto spares in ASHMORE to help you ask for DAVE ,PHIL OR MICK theirs a couple of fully qualified machanics working their ,, THEY WONT STUFF YOU AROUND.

 

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Posted: Wed Nov 16, 2005 7:53 pm 
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From experience (about 20 years ago!!) an intake backfire when you jump on the accelerator suddenly is caused by a suddenly extremely lean mixture, and this is made worse by the longer timing on the cam.
The most likely cause is the accelerator pump on the carby not working , or not feeding enough extra fuel to the air/fuel mix at the moment you press the accelerator hard. A cam with longer timing can reduce the vaccuum in the manifold at low speeds meaning the jets in the carby will be even slower to start feeding fuel to the fuel air mix when you dump the throttle, so you may need more fuel fed through the accelerator pump to make up for the lag.

Hope this confused explanation helps.

 

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Posted: Wed Nov 16, 2005 8:43 pm 
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i think spark try cap ect also did you get the dizzy in the right tooth?if all this is right rip the front off again and check your dots i know you cheched them but its not hard to get them a tooth out(ive done it)but im leaning towards the dizzy being a tooth out
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Posted: Wed Nov 16, 2005 8:45 pm 
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oops forgot the old crossed a couple of leads check that aswell it wont be nothing bad coz nothing stops a clevo
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Posted: Wed Nov 16, 2005 11:00 pm 
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Ok heres todays update,

I pulled the front of the donk & rechecked the cam timing with a degree wheel, I ended up advancing the cam 4 degrees as ford retarded the crank key 4 degrees to meet anti-pollution specs.

Better but still not right.

Next I ripped the rocker covers off and ground the valve tops.
When I put it together (prior to this prob) I noticed that the valves depressed what I thought was alot when I bolted down the rockers (more than a X-Flow which most of my rebuild experence is with).

Now one thing I remembered was a couple of years ago when I bought my XC the bloke gave me a card with the cam specs on it, at some stage I've misplaced it but I do remember the specs were hand writen on a yellow card. I maybee wrong but maybee it was a regrind and not a billet cam. If so I wondered if they put longer push rods in to compensate for the regrind?

Anyhow after grinding the valve tops and puttin' it back together it runs HEAPS better, still not quite right, but heaps better as in only a bit of backfire.
Quote:
From experience (about 20 years ago!!) an intake backfire when you jump on the accelerator suddenly is caused by a suddenly extremely lean mixture, and this is made worse by the longer timing on the cam.


This latest has bought me to another interesting fact. Now that it's runnin' a bit better I have noticed a HUGE flat spot in the carbi and getting through the flat spot is when the backfire happend.

Unfortunatly I don't know a lot about carbi's (damm modern EFI's), but it has a 4 barrel square bore Holly, dont know the CFM or the model No.
You can view here: http://www.geocities.com/xbgtford/Holly1.JPG
and here: http://www.geocities.com/xbgtford/Holly2.JPG

Hope that helps.

Hey 68keith, do you know of any good carbi joints on the Coast? There is one (I think) in Lawrence Dve but not sure?

Once again thanks for everyones help. :D :D

 

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Posted: Thu Nov 17, 2005 1:21 am 
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You should have used shims under pedestal or just shortened your pushrods.. Have you checked coil bind and rocker ratio etc?? I hope they are not std valve springs?
Again check accellerator pump .. There should be a screw with a spring on it.. Adjust this screw or you may need different cam is accllerator pump?

 

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