Engine: WhyChevy V8 only running on 5? - 10-Tenths Motorsport Forum

JohnHenderson · 11 Apr 2006, 17:25

I'm stuck and would welcome suggestions (I'm an amateur and I'm sure I've missed something obvious). I've got a small block Chevy V8 on carbs (on an F5000) which refuses to run on cylinders 5, 7 and 8. I only run it for a minute or two in case I damage it, but the exhaust pipes from those cylinders are (relatively) cool to the touch and it backfires a bit (burning fuel in the exhausts?). It's a 327 converted to 302 (4" bore x 3" stroke, apparently from a 1967 Camaro

ee David Vizard book), iron heads, Webber 48IDAs. So far I've checked: fuel (plugs are wet, but not soaking wet, good fuel pressure on guage), spark (plugs spark OK out of the head held against earth), plugs (new, easy start rather than racing), compression (high on all), valves (took a head off and valves/springs/pushrods/rockers/etc all OK). Finally,timing: here's where I think the problem is, but I've checked many times and everything seems to line up (at TDC:No1 both valves closed, No6 both valves rocking, dizzy pointing at No1) and swivelling the dizzy a bit either way doesn't seem to help). Any suggestions what I could try next (other than the obvious one: "Take it to a pro"!). Many thanks!

Al Weyman · 12 Apr 2006, 16:13

Do you have a spark at the cylinders that are not firing? What ignition/Distributor set up are you running?

StephenRae · 13 Apr 2006, 16:19

Presumably you've got the firing order right, it may be wetting the plugs..swop firing ones for those not firing or try heating them...you could try balancing the 'bad' leads on the plugs...this somehow intensifies the spark...I don't really know why!

Chris Harrison · 13 Apr 2006, 16:57

I am going to guess the problem is the balancing of the carbs.
With a setup like you have you are running 8 single cylinder engines loosely connected by the crank.
You will need to get a tool to help you syncronize the carbs and you may find that you have different air flow through the two barrels of one of the carbs, especially with the IDAs they have a very long throttle shaft and sometimes the shaft gets bent and the blades get out of balance.
Some carbs have a bypass screw to adjust indiviual flow or you may have to adjust the fitof the throttle blades or bend the shaft.
Haynes publishing has a good manual ISBN 1 85010 020 9 that will be a big help
Good Luck

Al Weyman · 13 Apr 2006, 19:51

He must have fixed it, he's disappeared!

StephenRae · 14 Apr 2006, 09:09

Maybe he got it running on all eight and the sudden surge of torque twisted the car off the stands and it pinned him against the back wall of the garage.

Al Weyman · 14 Apr 2006, 09:16

Ha Ha HA

JohnHenderson · 15 Apr 2006, 13:24

Ha ha! Very funny you guys! (especially Stephen's kind thoughts!!!) Sorry I didn't get a chance to get back sooner. Thank you all for your brilliant suggestions. I re-checked everything (swapped plugs around, which are new), even the bits I'd never touched, and found that the dizzy had two leads in the wrong place (not me guv! like that when I got it). No8 and 7 leads. So I swapped them to the correct firing order and expected everything to run sweet (well as sweet as a lumpy V8 ever sounds). But no such luck. It now runs on all but 5 and 7, the exhausts of which remain cool to the touch. There are sparks at the plugs, but it seems very little fuel on them (after the engine's run for a minute or two they should be soaked right?). As 5 and 7 share the same carb, it looks like the culprit, don't you agree? So I'm going to strip it (again!) and see if there's dirt in there somewhere and if everything works. It's probably more complicated than that (I didn't find any muck the first time I stripped it), as Chris suggests, but I'll try to eliminate the easier checks first. I've also suspended No7 plug lead away from the others, as someone told me crossfire was sometimes a problem with these two, as they're next to each other in the firing order on the dizzy. In response to Al's question, it's a regular-looking distributor (no name, looks old, ancient capacitor, but new-looking cap/leads/rotor and I changed the points) with a red Bosch (Australia) coil and ceramic ballast resistor. Now, if you don't hear from me for a while, don't assume I'm stuck under the brute... (although I have tried to start it in gear - a mistake you only make once... hopefully)
Many thanks again!
John

StephenRae · 15 Apr 2006, 13:46

Glad to hear you're still with us...how about swopping two carbs before stripping . then you'd know for sure...

StephenRae · 15 Apr 2006, 13:49

Glad to hear you're still with us. How about swopping two carbs before stripping. then you would know for sure...

Alan Raine · 15 Apr 2006, 15:22

You must have a big garage Steve - there's an echo!

norman-normal · 15 Apr 2006, 20:27

On a V8 the point gap is very critical, a slight amount of wear in the distributor shaft brgs can cause the gap to vary between cylenders....Also American cars have an external ballast resister, most other country's incorporate it in the coil, and for that reason, coils from most country's will not work well with American engines.

Al Weyman · 16 Apr 2006, 06:58

also and it is an easy mistake to make, on a dual point dizzy to set the static timing up on the trailing not the leading set of points, I know I only did just that the other week. To save any stripping why not getthe thing running and dribble a bit of petrol down the offending carb and see if it picks up on the cylinders (stick some fuel in an oil can and squirt it in is probably safest). Sounds to me like it may have a stuck carb inlet valve.

JohnHenderson · 16 Apr 2006, 18:21

More excellent suggestions there - thank you guys! I'll try dribbling petrol in the offending carb first (I never wanted eyebrows anyway) as Al's stuck valve theory would be a logical explanation for what I'm seeing (the plugs on non-firing Nos 5&7 should be soaked, right?). I'll have another look at the points and the static timing (the central shaft in the dizzy does seem to have a lot of rotational free-play - I can rotate it quite a lot - is it supposed to be like that?). I don't imagine there's much I can do if the dizzy bearings are worn is there (other than replace the dizzy)? Norman: the Bosche coil has an external ballast resistor and is designed to be used with one (says so on the side). Then I'll try Stephen's suggestion to swap the carb with another . Then I'll try Stephen's second suggestion to swap the carb with another (ha ha!) Then Chris's carb balancing check + book. Then I might strip it again. Then I might take up origami... I've got other stuff I have to do at the moment before I can have another go at the engine, and may be a while reporting back (I sound like Captain Oates)! Thanks again for all the suggestions! Much appreciated!

Al Weyman · 16 Apr 2006, 19:39

You should be able to rotate as that is the centrufugal advance weights throwing in, I dunno you guys that are bought up on ECU's and electronic ignitions, make sure as well the points are opening on the ramp as the dizzy is turning in the correct direction and not closing (i.e you have not static timed it assuming the thing rotates the opposite way, easy thing to do).

Welcoming AmericanLeMansFans refugees.