26 replies to this topic

[miniJJ]

Hi.

998 stage 1 kit, megajolt ignition, hs4 carb

My car has been having problems for a while now. On the way back from the mot centre a few weeks ago, we found out the fuel gauge wasn't working, and the car sucked some crap up into the carb. It was completly undrivable, revs screaming by themselves, then the car would die, backfiring masively and my dad had to basically get it home in first gear, the car oscillating betwean 5000rpm and 0 by itself.

It was so bad the mechanic at the local garge thought the timing was completely wrong. I have megajolt and confirmed my maps are good and working properly. I've switched betwean a previously working 998 stage 1 map, to a aldon yellow dizzy map, and now the default megajolt safe map.

I stripped the carb, and removed lots of silt from the float bowl and jet. After this it was in a semi driveable condition. It was still driving really badly, stalling / backfiring, a nightmare, I'd re set the fuel mixture and resetup the carb.

This is where I am now. I have replaced HT leads, redesigned my vr sensor mount for the megajolt to make sure it's rock solid, tried several known good megajolt maps and the car is still awful to drive. Now, you can drive it, but it badly hesistates, bucks and backfires under load, resulting in loss of power. I have to set a lot of revs when setting off to stop the car bucking and backfiring. With no load on the car, the car revs up fine with the very rare small pop of a little backfire.

I have pretty much eliminated a timing / ignition problem. This leads me back to fuelling. I rang the machanic who did the mot, and explained this. He thinks the carb is terminally damaged or another problem in the fuel system.

Any help? I really need this sorted for next week's half term!

Thanks, JJ

Edited by miniJJ, 28 May 2010 - 04:54 PM.

[01smartc]

Is the pump still pumping fuel effectivly, if you have had a load of *melon* through it it might not be. Pull the pipe off carb and crank it over, see if fuel pumps out of the pipe into a bucket or somthing. Eliminate that.

Whats it like with the choke on?

Edited by 01smartc, 28 May 2010 - 06:06 PM.

[miniJJ]

Yeah it's still pumping at a decent rate. Although I just noticed the new fuel filters I put on a few weeks ago are manky again! I'm going to fit new filters again tonight and hope that they took the last of the contaminants out of the fuel system.

could a partially blocked fuel filter could explain why it seems to hesistae and backfire under load?

Thanks, JJ

[Retro_10s]

And have you replaced the needle valve inside the float chamber lid? Is the misfire through the carb or the exhaust?

[miniJJ]

I have not replaced the needle valve, but I made sure the float and valve are operating correctly and are filling the float bowl properly. I think the backfire is through the exhaust, it's not like a huge bang (like it was 3 weeks ago) just a little pop; but inside the car it bucks and loses power during it.

Saying all of this, I've just replaced the fuel filters, and took her out for a drive. It drove the best it has done since the problems started. The bucking / backfiring only seems to occur under 2000rpm, (mind you, worse under load) after that the car is great! (although is it socially acceptable driving at 3-4k all the time? )

I'm starting to think the carb setup is out, and the ignition mapping could do with tweaking. Under 2000rpm the car feels underpowered and it feels like it's bogging down. After this, you can hear the engine note change and you can feel the increase in power straight away, the power it should have.

Do you think the mixture and timing need adjusting to solve this? or is it more likely another issue?

Thanks, quietly optimistic after tonights' drive.

JJ

[miniJJ]

Sorry Chris, missed you're question about the choke.

I haven't been using the choke, due to the warm weather and the fact the car starts fine without it. I'll use the choke tomorrow and let you know what effect this has on the car.

I've set aside tomorrow to drive the car under differant driving conditions and report back here with results. I'm going to see if I can get the fuelling spot on first (using the lift pin as stated in the trusty haynes) once the engine is nice and hot. I'll then drive it again and take it from there.

Am I on the right lines here? Or chasing the wrong problem...

Thanks, JJ

[miniJJ]

Took it for a drive earlier.

Under about 2.5krpm the car is very underpowered. It feels really rough, it's very slow and theres backfiring. When it's stuttering, theres a loud noise, it's almost like a click or crack, I'm not sure if this is a backfire from the carb? In my experience backfires from the exhaust are alot deeper than this sound. The problem is worse under increasing load.

Then once you hit 2.5krpm it's fantastic. Theres a very noticable change in engine note and you can feel the increase in power, it really pins you back in the seat. This is the power it should have.

I tested the mixture setup using the lift pin on the carb. The engine note remained constant, indicating a good mixture.

Do you think this is a carb problem or a ignition mapping issue? I've tried several known good igniton maps and no sucsess.

Help me please! I'm really gutted about this...

JJ

Edited by miniJJ, 29 May 2010 - 12:41 PM.

[miniJJ]

I'm currently using the bottom map, could this be the cause of the awful performace under 2.5krpm? Or does this look ok?



Help! haha

[miniJJ]

C'mon guys, can no one help?

Spent all day today on a quiet industrial estate trying to sort it out. I ran it with the choke and it felt alot better, pulled better, less stuttering and backfiring. So, I kept enriching the mixture until I found a good setting. (which I did, It ran perfect, best ever so far)

So, thinking it's problem solved, I drove up the long road of the industrial estate, accelerating and braking in differant gears; and I was soo happy to have no lurching, spluttering or backfiring. It felt great one minute, like it should; then I did a U turn at the bottom of the road, drove back the way I came and it was backfiring, (I think through both carb and exhaust) spluttering, lurching, no power... etc. again, worse under load. The problems seem to get worse after cornering, eg leaving a junction / after manouvers and going up inclines.

Also, idle speed changes by itself. After I adjusted the mixture, I corrected the idle spped to about 750rpm. Then 5 mins later I turn off the engine and restart the car, and the car barely strarts, idling about 500 and sounding unhealthy. After a few sharp revs idle speed picks up to normal; but every time you start the car it appears to be differant.

To me thats not ignition related. What can cause this? I have a full tank of petrol if it helps.

Thanks, JJ

Edited by miniJJ, 29 May 2010 - 05:19 PM.

[sonikk4]

Still sounds if you have muck in the fuel system. Take the tank out and clean it thoroughly. With the fuel line i would suggest if you have a compressor or access to one back blow through the fuel line to ensure any muck is blown out.

Put a couple of gallons of fresh fuel in and try again.

[miniJJ]

Would muck cause such a immediate change in how the car is behaving? EG straight after a U turn it's back to being awful?

Is the carb likely to be buggered too?

Thanks, JJ

[sonikk4]

A quick story many years ago my old man used to have a Hillman Minx. Some toerag stole her and he thought that was that however 2 weeks later it turned up but the retard chucked a load of muck in the fuel tank. We tried everything to clean it out but to not much success. Sometimes it would run fine then other times it ran like a piece of doggy do do's.

In the end the tank came out, flushed along with the fuel line, carb cleaned and she then ran like clockwork hence what i posted earlier.

Its just an idea and it certainly cured my dads problem.

[miniJJ]

Yeah, I'm starting to think that muck in the fuel system must be it. I couldn't believe how it ran perfectly for five minutes, did a U turn then it ran shockingly on the way back down.

How would I go about cleaning the mechanical fuel pump? Worth doing I suppose.

Thanks mate, I'm off to try and drain my petrol tank (only filled it yesterday! )

JJ

[miniJJ]

I'm getting really disheartened with it.

Drained the fuel tank, took it off and found about a cubic foot of rust and grit inside it, so much so, that the remaining petrol wouldn't drain out of the outlet by itself, it was hugely blocked!

Completly cleaned the inside of the petrol tank, blew compressed air through all the fuel lines, then refitted it, filled her up with clean petrol, and went for a drive this morning. It drove fine, no backfiring at all, but the carb lift pin indicated it was running too rich, I weakend the mixture until the pin test was sucsessful and the car seemed fine with the change.

Went to drive it this afternoon and it was backfiring and losing power under load. It gets alot worse after sharp cornering or a U turn (was back on the industrial estate trying to figure out whats wrong) Whenever I've checked the huge inline filter before the carb is completly full of fuel, and I checked the fuel pump is pumping, it has good pressure; so I think theres still fuel going to the carb.

Help!

I'm about to completly give up. This has really upset me after paying out over £1000 this month to repair, mot and insure it. I'm gutted, I'm getting nowhere, and I can't drive my car properly. (Parents are sick too, want me to get rid of it and buy a new car, they've had enough already)

JJ

[miniJJ]

Double post deleted.

Edited by miniJJ, 30 May 2010 - 06:51 PM.