help....spark or fuel problem

[Dreco]

hey all, My engine is hesitating between 5000 and 6500 rpms. can anyone tell me what to do, to determine wether it is fuel or a spark problem?? My motor runs sweet at the lower rpms, pulls hard, but when I hit around 5000 rpms or so it starts to hick-up (cuts in an out), on its way to red line. Is it starving for fuel? when I excelerate slowly it doesn't do it. Anyone have any suggestions?? I've set my timing, I've adjusted my valves (I'll check the clearance again if need be) I've balance my carbs. My plugs look good. I'm baffled.

[Dreco]

correcton.....It does it whether I excelerate slow or fast. I'm going to play with the timing again and see if that's what's going on.

[2ManyZs]

My first idea is either your float setting in the carbs or maybe your mixture is off just a little.

With a bored and stoked set up like you have, you're gonna need to be a bit richer than most other engine set-ups.....

That's where I would start anyways....

[hmsports]

What about fuel pump? At higher RPMs the carbs need quite a bit more fuel. Maybe the pump isn't keeping up?

[Dreco]

My gut tells me it's fuel. I just got done taking it for some test runs, and everytime aroung 5000 rpms or so it would sputter then fire, sputter then fire as I went up from 5000 to around 6500 Rpms. there was never any smoke. If I backed off the throttle a little it would stop. Would running too lean cause this??? I think I'll start with increasing the float level. I've often thought that the mechanical fuel pump may not be adequate. How much psi is the stock pump rated at?? I was told by rebello that my timing should be set at 32 degrees at 4500 rpm, and it's dead on. oh that reminds me, I can take engine, in neutral , to red line, no problem, it's only when a load is applied. ( if that helps) I DO have a bigger cam (490 lift). I'm thinking that when my cam really starts to engage, it just starts to really suck the fuel, and there isn't enough there.

[Daniel]

The first item of course is fuel filter or filters.I had to say it.The next item is the air filter.I had a friend that his foam air cleaner would restrict flow.A test without would verify that.Plug gap incorrect I have also seen cause this.Sorry for so little help.Has the car ever run correct or is this new?What changed?

[Dreco]

all the filters are fine. I'm going to start with the carbs first. It hasn't always done this, but then I don't take that high in rpms THAT often.

[Doehring]

Hi all,

my stock '72Z has the same problem and up to now I couldn't find a solution. Whatever I did, it became worse. Timing, gap, plugs, everything is alright. So I cleaned the carbs, found the choke sticking and made it work properly, the carbs are balanced, the float level and float bowls (see my gallery) are set, still problems above 4500 rpm under load. I started experimenting with the needles. In the end at about 4500 rpm both carbs started backfireing and black smoke came out of the exhaust at the same time. One explanation could be that the mixture is too fat on the one hand and that at the same time there is a lack of fuel for milliseconds (not enaugh fuel ?). So I will try to go back to the original setting of the needles/carbs and start to study the ztherapy video that I got today (Nice people there at ztherapy!). Nevertheless, my feeling is that the fuel pump or the lines make the problem.

Drecord, let me know what you find out. I hope we all together can find the right way what to do. Thanks to everybody for the input on that.

Rolf

[Dreco]

I looked at a second set of carbs that I have ( from my original engine) last night. I noticed that the piston in my spare set has a better seal than the set I had bored out. I'm wondering if the vacuum of my engine can't raise the the piston fast enough to let the correct amount of fuel up through the nozzel at higher RPMS? Anyone have thoughts on this??? I'm guessing that would be the same scenario, if you had too thick of an oil in the carbs. Speaking of oil, what weight oil should you use in the carbs? I know it's supposed to be pretty thin. I used some 5w-20 is this thin enough?? I figure I'll just swap some of the carb parts and see if there is a change.

[biker]

Just spent the weekend at the track but on getting there the car was running like a dog.

As soon as it hit 4500, the thing would start to backfire and not rev any further.

After changing plugs, points, condenser, fuel mixture, fuel filter and exchanging a few choice words, someone kindly lent me a dizzy.

Thing went like a rocket up to 6000.

It appears that the counterweights in the Dizzy might be jammed and not allowing the mechanical advance to work properly.

Check that. There is another thread here about Dizzy problems as well

Biker

[Dreco]

I fixed the problem. Seems I didn't advance my timing enough. I turned the distibuter a little, and ZZZZOOOOOOMMMMMM!!!!!.......Daddy Like!!!!