A7 misfire

[BSA500]

As the title suggests some misfiring issues. Its an alloy headed,8.5:1 compression with B6ES plugs(b7 kept fouling and failing) Mainjet 240,needle 1065, slide 3 1/2. She starts fine now ticks over since changing the plugs but used to bog down and spit when pulling away, sorted that by raising the needle but that is now on its richest setting. Under accerleration(sp) ie 1/4 open and above misfires a couple of times up the rev range enough to effect a decent blast. My question is what is the best way to go increase the needle jet size(107) or richen the slide(3) or both or none help!

thanks Andy

[A10Boy]

Perhaps a Colortune would help here?

[BSA500]

The colourtune would not be helpful as she revs out fine while parked and only really misses under load. Had a check and it already had a 107 jet so swapped to the 1065 and it was much worst. Took it out for a spin and it seems to stutter all the way from 1/4 throttle like hunting but not if you know what I mean innit

Andy

[MG]

The settings you are using sound very rich. Could there be excessive wear on the slide/carb body?
The float level and unristrected flow through the taps and pipes (incl. filters) are other things to check if you haven't yet.

Cheers, Markus

[BSA500]

Those settings are pretty much what BSA specced for the A7s. The slide isn't sloppy and petrol flow etc are fine not sure if the problem is a rich issue or weak. I am guessing at weak and was wondering what people find works better richer slide or needle jet. Problem is according to the amal tuning chart the slide and needle jet control the same range 1/4 to 3/4

[a101960]

The colourtune would not be helpful as she revs out fine while parked and only really misses under load.

I would not be so sure about that. The problem you describe is very probably caused by faulty spark plugs. I once did a service on my car which included fitting new plugs. I had exactly the same symptoms that you have. The plugs that I fitted were brand new Bosch plugs. I assumed wrongly that because I fitted new spark plugs that the plugs could not be the source of the problem. On re-fitting the old plugs the engine ran perfectly. So my advice would be to try another set of plugs. Also check that the copper conductor wires have not corroded at either end of the HT lead. Look for a blue/green discolouration especially where the caps screw in.

John

[BSA500]

Old plugs failed so new ones in(B6ES) and beside this issue runs much better actually ticks over now. HT leads are new Copper core.
Andy

[chaterlea25]

Hi Andy,
Have you checked the fuel height in the carb, This is the first check to do
Does it have a monobloc carb fitted?
Does your carb have a choke slide? if it has lower the slide 1/2 to 2/3 and test under acceleration
if it improves then try a cutaway 1/2 or1 number down
Another trick is to richen the pilot some more and see if the hesitation reduces or dissapears?
The effects of the different partsof the carb have on performance/tuning overlap each other

HTH
John O R

[a101960]

Andy, When I had the problem I described with my car the plugs were brand new. I could rev the nuts off of it and it idled perfectly. It was only when I went to drive and it was under load that I had the problem. Just because the plugs are new it does not mean that they are not faulty. By the way I use champion plugs. I have used NGK plugs but for some reason in my experience they seem to have a very short life. As a matter of interest how far have you got the pilot screwed out. About 1 1/2 turns is about right. I set my carb initially with a colortune. I found that the carb was to weak after setting up with that. It was an absolute pig to start, but seemed to run ok once it was warmed up. By setting the pilot screw to 1 1/2 turns out it cured all of that and is now a first or second kick starter both hot and cold. By the way my plug colour is coffee coloured in the centre and black around the outer edge. The engine pulls like a train. Spitting back could be either a weak mixture or the timing slightly over advanced. One other thing. Check the integrity of the carb to manifold seal. With the engine running squirt some WD40 or similar on the joint to see if it gets sucked in.

John

[BSA500]

OK I had typed this all out last night but deleted by mistake so I will try again...

Carb manifold sealed no leaks
Pilot setting 1 1/2 not smoking thats fine first kick start hot/cold
Sure its not float level checked that
1" choke correct for A7SS spec
240 main runs fine full bore
3 1/2 slide no sloppyness
choke slide well retracted
needle 107 will not run well with the 1065

checking the tuning guide suggest slide and I quote the exact text as this seems to fit my problem...

if the engine jerks under load and there is no spitting either the jet needle is to high or a larger cut-away is required to cure richness

so I will lower the needle a couple of notches and if no improvement a larger cutaway slide
will report back

[BSA500]

So raised the needle two notches(middle position) and it needs to be warm for a clean getaway off the pilot but it still jerks so would I be right in trying a no4 cutaway slide and tuning the carb into that ?
Andy