5K Dies Under Load

[GJM85]

Yeah it's never run right since it was assembled. This is the first engine i've ever built and I thought I had it nailed. Yeah it blows back out the carb. Which makes me think its sparking on the intake stroke but with the timing spot on it continues. Thus leaving the camshaft... :(

[altezzaclub]

I don't think so. What is a tooth worth in degrees?? You should have the inlet closing 65deg after BDC shouldn't you, and it remains closed as the piston compresses to TDC and fires, then the exhaust opens 65deg after TDC. So there's not much chance of flames shooting back up the carb if its a tooth out.

 

What about tappet gap?? Does it read the same as you set it if you check it a day later? If there was a problem in there it might hold the inlet valves open to let it backfire. Not very likely....

 

Head gasket?? Flashing between two cylinders is the most likely thing I can think of. Try a leakdown test- Hook a compressor up to a compression gauge fitting and pressurise one cylinder at a time. We did one on my Altezza a few weeks back and it showed leaking exhaust valves straight away.

 

 

[Evan G]

if i remember correctly theres 18 teeth on the crank and 36 on the cam gear. so 1 tooth on the cam is 20 degrees?

[GJM85]

if i remember correctly theres 18 teeth on the crank and 36 on the cam gear. so 1 tooth on the cam is 20 degrees?

 

yep, 20 degrees of crankshaft rotation.

What if the blow back is occuring on the overlap of the cam? The 50 degrees of crankshaft rotation that both valves are open... between the exaust stroke and intake stroke.

Edited November 16, 2011 by GJM85

[GJM85]

If the cam was installed just 1 tooth advanced then the cam would be functioning with corked durations. On the camshaft overlap the intake valve would be opening 35 degrees before tdc and the exaust valve would close at 15 degrees after tdc.

This means at tdc between exaust and intake the intake valve is further open than exaust valve. The uneven overlap could be causing exaust gases to be forced back through the intake causing the blow back.

 

NB: when the blow back occurs it is not ignited(flash back).

[Evan G]

please don't tell me your running a stock exhaust system?

 

edit - if you remove the studs from your timing case cover it shouldnt take long at all to remove and check cam timing. but then again you can check cam timing from the push rod

Edited November 16, 2011 by Evan G

[GJM85]

please don't tell me your running a stock exhaust system?

 

no, 4-1 wrapped headers and a 2" system

Edited November 16, 2011 by GJM85

[GJM85]

If the cam was installed just 1 tooth advanced then the cam would be functioning with corked durations. On the camshaft overlap the intake valve would be opening 35 degrees before tdc and the exaust valve would close at 15 degrees after tdc.

This means at tdc between exaust and intake the intake valve is further open than exaust valve. The uneven overlap could be causing exaust gases to be forced back through the intake causing the blow back.

 

NB: when the blow back occurs it is not ignited(flash back).

 

In fact if the above was the case just 1 tooth would mean on the camshaft overlap the intake valve would be opening 45 degrees before tdc and the exaust valve would close at 5 degrees after tdc. Because 1 tooth on the cam is equal to 20 crankshaft degrees.

 

This would also be why it idles best a 20 degrees of timing...

 

Now why would the exaust gases choose to leave the intake? Because once the manifold vacuum drops below the exaust back pressure than the gases will naturally take the easist exit. That is why the blow back only occurs under thottle.

Edited November 16, 2011 by GJM85

[altezzaclub]

NB: when the blow back occurs it is not ignited(flash back).

 

Ah, OK that is different.

 

In fact if the above was the case just 1 tooth would mean on the camshaft overlap the intake valve would be opening 45 degrees before tdc and the exhaust valve would close at 5 degrees after tdc

 

Dead right, and that would push exhaust gases up the intake as the piston covered that last 45deg.

 

So, Bluetack a circular protractor onto the crank pulley and watch the valves.

Edited November 16, 2011 by altezzaclub

[GJM85]

I got out to my dad's where the 20 is today and had fiddle...

 

I cranked the engine around to TDC #4. Using a razor blade I leveled the valve retainers of #1 to check the overlap of the cam. This is what I found.

 

To me the cam appears to be about 5 degrees retarded. Could this be the cause of my problems? I ask because 5 degrees on the crank is only 2.5 degrees on the cam or 1/4 of a tooth.

Edited November 19, 2011 by GJM85

[Evan G]

how does it rev up high? it should pull like a school boy in the higher rpms.

 

advanced cam means more torque

 

retarded means more hp/revs

[GJM85]

Yah when I had the big 38/38 on it once it got over about 3000rpm it really started to go, but backing off for corners and getting going again was nothing but trouble.

 

Here's my interpretation of the cam in microsoft paint. First how it should function. Second how it is functioning.

 

Oh, and there's lunch...

Edited November 19, 2011 by GJM85

[altezzaclub]

Well, what you have drawn there should make it rev better- the later the inlet closes the higher it needs to rev to get the cylinder filled.

 

As you said, 5deg is neither here nor there really in a road car, you can't adjust it a 1/4 tooth.

 

Must be something else! Electrical insulation breakdown under load or fuel not right as the throttle opens...

 

 

 

 

Can't go wrong with XXXX gold! :laff:

 

 

[altezzaclub]

The last compression test I did ranged from 140 to 170 psi

 

Try that again, they should be closer than that.

 

Chase the headgasket with a leakdown test. Just hook a compressor up to a screw-in compression gauge tube and pump 120psi down each cyl one by one at TDC. You might have it leaking between cylinders, but not into the water jackets or oil, so it fires a wrong cylinder occasionally and doesn't have compression for power.

 

Tappets gaps OK, the same as when you set them??

 

I found out why my Altezza won't start, after 4months of chasing all sort of possibilities, it has carbon lumps sitting on the exhaust valve seals & seats, so no compression... Lousy fuel the last time it filled up is the theory... $1000 if I do all the labour!

 

 

[GJM85]

All the electrics are new. The head was off in September when I changed the pistons. The head gasket was fine then and it was suffering with this problem before. I can't help but feel that this 5 degrees is my problem and the reason I can't get the timing right. I've addressed every other possibility...