perhaps a carburation problem ?

simontwithers · Post by **simontwithers** » Fri Aug 28, 2009 9:37 pm

Hi,
heres 1 for those who like a challenge..

I have a 3.5 SD1 engine that was dry stored for a number of years. I now have the engine installed and running but im struggling with a big hole as i pull away from a junction. It pulls off and then half way through first gear it bogs and then immediately kangaroos forward and then i change into second gear.

This under normal driving conditions.

the engine has a mild kent fast road cam, and holley 390 vacuum secondary carb, otherwise its standard.

I have the timing set at 14btdc - set at idle with vac disconnected.
the holley has a full rebuild kit in it.
i set the secondary idle at around 1/4 to 1/2 turn open.
i set the float levels as per holley instructions

and set the idle mixture screws at 1.5 before closing slowly till the engine stuttered on each side at idle and then 1/4 out. (i have now added a further 1/8th turn each side to smooth things out after road testing)

the holley is as bought from real steel with standard vac secondary spring, the main jets have 512 stamped on them, although the 2 is stamped backwards.

6.5 powervalve

is a list 8007 carb

vac gauge shows 16 at idle

new plugs and leads
new dizzy cap and rotor arm
12v coil with no ballast resistor
running super unleaded.
advance springs free and vac advance diaphragm works when i suck on it

I've played with the timing - on one occasion i ended up with 20 odd degrees advance at idle which got rid of the flat spot off idle and made it run so smooth with no pinking (20mph, top gear, foot on floor, up hill)

although it ran nicely i put it back to 14btdc so i can find the real cause of this problem and not just mask it.

this engine ran great before it was removed and stored, but now it has no power (runs out of steam on long hills and requires a change down) is also only returning 10mpg on a steady run

when cold starting, the manual holley choke just kills it dead so i assume its running rich. I start it with 2 pumps of throttle then fire it up, starts straight the way then i keep it ticking over with the throttle till warm enough to idle.

i've compression checked it at around 140psi on all cylinders and it runs cool.

please help me find the cause. does the fact that running lots of advance cures the problem suggest anything ??

i've included as much info as i can but please ask questions to help me get this engine running smoothly.

also i dont think the vac secondaries are opening, they are free to move but i put a paperclip on and thrashed it and the clip hadn't been pushed down the shaft ??

i also checked for vac leaks with wd40.

kind regards

Simon

just pulled the spark plugs after a 100 mile motorway trip and they are perfect, a light tan with no signs of overheating

ppyvabw · Post by **ppyvabw** » Fri Aug 28, 2009 11:46 pm

well these things are often difficult to diagnose but if you adjusted you base timing and it fixed the problem then maybe it's the advance curve of the dizzy. Some weaker springs (I think) may work. What dizzy you got? If it's a dual point one then I can gaurantee that's your problem lol. I had one and quickly learned my lesson

You say the engine ran fine before it was dry stored? Was it running with the same fuel pressure before? A dodgy float valve and higher fuel pressure maybe? That would explain your high MPG too and the fact that the choke kills it.

As for the vac secondaries.... I don't think they will open if you thrashed it whilst standing will they? Or did you thrash it whilst driving? In either case, I don't think the secondaries would cause the off idle problems you're experiencing. They shouldn't be open under the conditions you're describing i don't think.

simontwithers · Post by **simontwithers** » Sat Aug 29, 2009 9:02 am

thanks for helping me out.

the dizzy is standard sd1.

the fuel pressure is a steady 3.5 psi.

i drove the car when giving it some stick including foot to the floor on a long hill at high and low revs, im sure this should have opened the secondary's.

i now know the mixture is perfect at cruise from the plugs, but i have this problem off idle.

ppyvabw · Post by **ppyvabw** » Sat Aug 29, 2009 2:57 pm

Not sure about Holleys but if the secondaries are not opening and you have an off idle problem, could it be the diaphragm in the vacuum thingy that operates the secondaries? Maybe it's split, letting air in and also not allowing the secondaries to open.

Ian Anderson · Post by **Ian Anderson** » Sat Aug 29, 2009 4:04 pm

I have hear of these diaphrams failing after a single backfire

Ian

simontwithers · Post by **simontwithers** » Sat Aug 29, 2009 6:08 pm

completely stripped the carb today and blew the ports and jets out (again). All of the diaphragms were ok.

this is proving to be an elusive problem but im now convinced its the carb.

bigaldart · Post by **bigaldart** » Sat Aug 29, 2009 6:23 pm

If its rich after storage check the cork seal on the vaccuum passage to the secondary diaphram, other possibility is the power valves, try a new one and see what happens. You can test by sucking against the diaphram if you have good lungs. It should be closed with vacuum on it, then open as manifold vacuum drops, power valve that is.

Alan

simontwithers · Post by **simontwithers** » Sat Aug 29, 2009 7:03 pm

holley has been completely rebuilt. Is there any tuners in the midlands who offer a service to set up the holley professionally ?

Im happy to admit that i have reached my limit with what i can do with it unless someone here can tell me how to solve my problem.

i've done float levels, idle mixture, stripped and rebuilt the carb numerous times, played with ignition, checked plugs and fuel pressure, tuned carb with vacuum gauge, checked acc pump is working....

new ignition parts.

im 99% sure its a carb fault, even if its just getting it setup right. I know my main jets are ok because of the coulour of my plugs after a long motorway cruise and also because this engine ran great before it was stored.

now im out of ideas.

just a summmary of the above - i have a gutless v8 with a really bad bog when pulling off.

please help.