Engine will only run above 4000 RPM. Carbs or timing?

[Stanley]

What's the fuel pressure? Maybe the regulator crapped out. SU's like 3.5 psi. Maybe your other carbs like less? Too much FP could overpower the float valves, flooding it out until you get to 4000 rpm, maybe ok then.

Just a shot in the dark...

[bartsscooterservice]

I had the vac advance plugged with triples and with the SU's I've had it with a hose to vacuum and plugged, it made no difference. It's plugged at the moment and will stay that way, one less variable.

When the triples were on all the butterflies were in sinc.

The SU's butterflies appeared also to be in sinc before I installed them.

Just cranking the timing mark on the pulley is about at 10* with the timing light.

Zed, your right it is all over the place, but people make a suggestion, I go and try it and report back, any one of them might be the fix.

Demon Squirrels...THAT'S IT!

fresh fuel going to both carbs, and yes that is a new hose and it's feeding the vacuum tester.

I don't know what fuel pump he was using but the 1.5L per min. pump that I have now is enough for an idle.

What has changed? He had a header on I have a bored out cast iron exhaust manifold, he was using a generic HEI module and a Bosch coil, I'm using a matched HEI module and coil from Pertronix. He had the triple carbs which I tried extensively, but spark plugs were always wet and fuel spilling out the front, since installing the SU's no wet plugs and only a bit of gas coming out the front when it back fires through the carbs on shut down.

A little success today, I managed to get a very rough and low idle out of it twice, each time only lasted about 10 seconds and the timing light showed pulley mark at about 12* In between those 2 attempts were lots start and die immediately and the usual start and surge up and down like in the second vid. I changed nothing between these attempts as I was trying to get a reading with the timing light.

Chris

The wet plugs on the triples and the more dry plugs on the su's, tell something about a fuel issue though. But wet plugs also means way to much fuel on the other hand ( or lack of ignition ). I don't know about electrial fuel pumps in combination with carbs though. Have you tried the stock mechanical one with the su's ? How are your spark plug cables?

I still think it's the carbs though, they can come very accurate. Ask the previous owner what fuel pump he used on the engine with the triples.

[Travel'n Man]

Unplug the vacuum advance / fuel pressure 3 psi (+/-.5 lb psi)

Check your fuel filters to make sure there is no debris in them at all - I run two - one in the back before the pump with a metal filter - one inline in the engine compartment.

1000 RPM @ 12-14 degree - base setting

3000 RPM @ 36 degree - do not exceed

4000 RPM @ 38 degree - do not exceed

I set my timing at 3000 RPMs - I only use the 1000 RPM setting to get the car running - you must unplug the advance hose. This is my timing with my Mikuni's - this setup came from Rebello Racing and it was dead on the first time - give this a shot and see if it helps

Edited May 8, 2013 by Travel'n Man

[LeonV]

Okay-here's a thought. You say you changed exhaust.Now you know it's imperative that the the flange thickness on the intake and exhaust are equal. If your 'new' exhaust for some reason has a thicker flange--- then your intake is not getting torqued down properly and causing major vacuum leaks along the bottom.

I'm with Steve, sounds like a big vac leak to me.

[madkaw]

I remeber Chris's post about his header-looked almost homemade. I imagine flange thickness might not be the same as the stock manifold.

I have seen stepped washers to correct this, but that might be the problem here. I can only see a larger vacuum leak causing such a surge.

[bartsscooterservice]

It must be carbs, carburetors don't like fuel pressure, he's putting a fuel pump for injection engines on a carb system... Carbs don't like to much pressure, or else it will force more fuel through the needle and the fuel bowl wil overflow and drown the engine. That's what he's reporting.. soaked plugs and fuel dripping out of the carb.

Try a stock mechanical fuel pump...

I don't believe it's engine timing. And a 4 stroke engine can run without an exhaust manifold, so that doesn't matter really.

[grannyknot]

Okay-here's a thought. You say you changed exhaust.Now you know it's imperative that the the flange thickness on the intake and exhaust are equal. If your 'new' exhaust for some reason has a thicker flange--- then your intake is not getting torqued down properly and causing major vacuum leaks along the bottom.

The exhaust manifold is a stock unit that I have bored out a bit, the flange thickness is the same as intake manifold,

with new 1/4" thick washers on a new gasket, tight as drum.

What about a blocked exhaust. Above 4000 it may be able to push through enough exhaust to run. Try cracking the down pipe and see if anything changes...

Great idea, had hopes for this one, ran a drain snake up the exhaust hoping I would brake up the demon squriels nest then I unbolted the down pipes at the exhaust manifold, it sounded good but changed nothing.

grannynot,

im waiting on tires that are back order. so i cant drive mine at the moment.

mine is running well now. could you make use of my coil or anything for trail purposes?

Thanks, do you have a set of points? That might be worth a try.

What's the fuel pressure? Maybe the regulator crapped out. SU's like 3.5 psi. Maybe your other carbs like less? Too much FP could overpower the float valves, flooding it out until you get to 4000 rpm, maybe ok then.

Just a shot in the dark...

My last test of the pump was 1.5L per min. at 3.5 psi. I'll try the regulator at 1 and 6 psi in a fill test to rule that out.

The wet plugs on the triples and the more dry plugs on the su's, tell something about a fuel issue though. But wet plugs also means way to much fuel on the other hand ( or lack of ignition ). I don't know about electrial fuel pumps in combination with carbs though. Have you tried the stock mechanical one with the su's ? How are your spark plug cables?

I still think it's the carbs though, they can come very accurate. Ask the previous owner what fuel pump he used on the engine with the triples.

I have a mechcanical pump from the original engine, certainly worth a try. Plug cables are good.

I'm with Steve, sounds like a big vac leak to me.

I wish, that would be so easy fix.

I gotta say, this engine runs like a champ from 3000 - 4000 rpm, if only I could get it a little lower.

[Wade Nelson]

Several people are saying "big vacuum leak". Since this was a swap, what about a brake booster with a holy diaphragm.

How about plugging the vacuum line to the brake booster just to test. Take 30 seconds.

And no, a "Holy Diaphragm" is NOT what Catholics use for birth control.

[madkaw]

He's already eliminated that.

[bartsscooterservice]

And did you find anything so far?

[grannyknot]

I found that the fuel regulator was having no effect when I did the fill test at different settings, 5lbs or 1 lb the I'm getting 1.5L of fuel per min. The pump only pushes with 4lbs of force but to eliminate the fuel pressure as a possible problem I disconected the power to the pump and tried starting the engine with just what fuel was in the bowls, all the symptoms are the same.

Thinking I might pull these SU's and pack them off to Ztherapy so I can start working on the car again.

I'll let you guys know what happens when I get them back, thank you very much for all the help.

Chris

[Stanley]

But it was doing the same thing with the other carbs, right? When you swapped the carbs you'd have to swap the linkage too I guess, so that rules out the linkage or the butterflies up to some weird trick. Most auto parts stores should be able to get you a set of points in a day or two if they don't have it - at least you could rule that out. Is the gas tank full? Have you tried it with the gas cap off ?

I got nothing. That video is weird. Whoever figures it out should at least win a cheeseburger or something.