Misfire in 4th gear continues...

[dhenry]

Hello,

I have been having some issues ever since i bought the car a few months back. It is a 66 morris with a mk2 998 engine and box running on points and dynamo.

I believe I have an ignition issue. I will describe the symptoms and hopefully somebody can advise me.

The engine revs fine and functions normally through 1st, 2nd and 3rd gear, but in 4th gear the problems start. If I accelerate at WOT the revs will increase without incident, if back off the throttle to a steady cruising speed the engine splutters and misfires. This occurs at 3500rpm precisely. I can cruise at 3400rpm and the engine is totally smooth, but if it creeps up to 3500rpm it splutters and hesitates. If I then floor it the problem disappears and it will accelerate on up past 4000rpm without any hesitation.

I thought the ignition timing might be to far advanced and that with the high manifold pressure at cruising speed the vacuum on the dizzy pulling full advance was causing the issue. Then when I open the throttle the manifold pressure drops backing off the vacuum advance and returning the engine to smooth running. That was my theory. However, after retarding the timing by about 2 degress the problem got worse. The misfire then occurred at 3000 rpm in 4th not 3500rpm. So I returned the dizzy to its original location and tried to advance it 2 degrees and the same thing happened, the misfire occurred earlier at 3000rpm and the engine had noticeably less power. Finally I returned the dizzy to its original location and disconnected and plugged the vacuum hose only to find that it made no difference whatsoever. Again, around town in 1st, 2nd and 3rd it is perfectly nippy with smooth response.

I have checked the points and condenser and they seem to be functioning properly. Could it be a broken dizzy or might it not be the ignition at all? Carb? Fuel delivery?

Any suggestions would be much appreciated. As although I dont drive the motorway often at present I am stuck in the slow lane. Cheers,

Daniel

[Smiffy]

Condenser had a similar thing happen on the way to stanford, missing at 4500rpms on the way down. Got a new condenser at the show fitted it and on the way back was ok at 4500 but the was missing at about 3000. Got home changed it again to a old one out the garage and that sorted it. Apparently there all made in china now and shite.

[dhenry]

Yes, I have heard about the varying quality of condensers now. I will pick up a handful of condensers just in case. I was also wondering if the advance mechanisms inside the dizzy might be stuck or worn out causing it to be at full advance all the time or not advance at all.

[cheleker]

Borrow a vacuum tester. Remove the dist. cap. Hook up the tester. See if the top plate move. If yes, OK. If no, vacuum bad or plate stuck. Also, pull off the top plate and check springs and freedom of movement of the advance weights.

[Smiffy]

Yep bob weights could quite easily be gummed up and in need of a little attention. Cheaper to check these before trying to buy a new condenser. Take the dizzy out, take the base plate off, put the rotor arm back on. Then get hold the bottom and turn the rotor arm. You should see the bob weights move out, they should move all the way out to the stop pin. If they don't give it a good clean out until they move freely. I usually use standard thinners and a paint brush to clean out all the oil/grease & dirt. Then oil the weights and springs and put it back together, refit to engine & reset your timing.

[mk1]

I too suspect that all is not well inside the dizzy from a bob weight / spring perspective. I find that the best way to tackle mysteries like this one is to start at the begining & work through checking after each stage. I would do basic dizzy function, advance retard inc vac advance, if that all checks out OK then move onto condenser, points, rotor arm, coil, leads, plugs. After checking, fixing or swapping each component have a good test run & note the differences. If ALL the above are OK then move onto the fuel system, same method as above.

Not the quickest method, but thorough & more or less fool proof.

[dhenry]

Thank you for your replies. So I am going in the right direction?

I believe that the vacuum is functioning as my idle dropped after disconnecting and plugging it. So I think it must be the mechanicals. The cap, rotor, points, condenser, leads and plugs all look brand new so i suspect they were changed before i bought the car.

I will tear down the dizzy this weekend and set it up from scratch. Cheers,

Daniel

[dhenry]

Hello,

I had a go at the dizzy on Sunday. I took it apart and gave it a thorough clean. Everything seemed to be there although I have never rebuilt a dizzy before so I'm not sure what to look for. The weights and springs all seemed ok and move freely. The vacuum advance was knacked. I rebuilt it and set the points gap and set the ignition up from scratch. It seemed ok, engine was smooth and revved well. Idle was steady at around 900 but as soon as i tried to drive it I knew it was buggered. It was actually gotten worse since I cleaned it. It won't rev past 2500rpm at all now. If I add anymore advance the engine dies and wont start if I retard the timing it looses all power and stalls when I lift the clutch. Anyway, this morning i said f**k it and ordered a brand new high performance dizzy with electronic points and a tasty advance curve. Let's see if that does the trick. (I just wanted an excuse to buy a nice dizzy anyway).

Cheers for the help guys.

Daniel

[mk1]

I hope that your new dizzy solves the problem. May be worth checking the fuel system while you are waiting though.

[YMJ]

In 1991, I spent several hundred pounds worth of time, new lumention and carb parts before finding a new rotor arm at 99p would have done the trick. 'Worth checking first!

[dhenry]

That is good point. However, I did confirm the the vacuum advance was broken during the strip down and I knew one of those is £35 i think. This way I have upgraded my advance curve, replaced my vacuum advance and changed to electronic points all in for £65. Not bad value and as I drive this car to work everyday I think it will prove to be a worthwhile investment. When all this is sorted i should see some improved performance and MPG. Cheers.

Daniel

[dhenry]

Wow, the new dizzy has made an enormous improvement. I have a new car. It has so much more power than it ever did. Loads of torque and power on tap whenever I want it. The drive to work this morning was a lot more fun. I am really glad I changed the dizzy, however, I still have the misfire on the motorway. Which just leaves one thing. Fuel pump. Hand's up who said fuel system.

Currently the car has one of those cheap electric fuel pumps. The little black inline ones. I would like to upgrade to a facet cube type pump and mount in the boot. My question is what psi do i need as they come in everything from 1.5psi. At the minute I am running a single carb but I plan to run twin 1.25's soon with a ported head and cam upgrade. I was thinking somewhere in the ball park of 3-4 psi. Any suggestions? Cheers,

[dhenry]

Hello,

So the misfire still persists. I have replaced the entire distributor, points changed to electronic, new cap, new rotor, new vacuum hose, new fuel hose, new fuel filter, new facet fuel pump rated at 4.5 psi and still the car misfires at around 3000rpm in 4th gear only.

What to do next. The car has a hard fuel line that runs along the underside of the floor. I was wondering if perhaps this was somehow blocked or dented causing fuel flow to be reduced. Would it be worth me replacing this length of hard fuel line with a rubber fuel line? At least the rule out any blockages.

The only parts of the ignition I have not replaced are the plugs and the coil. Shall I replace those to rule out ignition entirely?

The only thing left I can think of is the carb/intake. I plan to upgrade to twin SU's anyway so is it worth going ahead and changing carbs? Alternatively has anyone local got a spare single SU and intake manifold setup for a 998cc that they might lend me for a few days so I can potentially rule that out or discover that I have a shitty carb?

Let me know your thoughts and thanks for the help along the way.

[mk1]

I will have a very reasonably priced pair of Burlen HS2's available very shortly.

But in the meantime, have you managed to ascertain whether its going weal or rich at 3,500? Pulling the choke out will tell you this, if it gets worse then its too rich if it gets better its too lean. Coils are pretty poor nowadays, it could be that.

It is very hard to diagnose problems like this long distance, especially when all the "usual" things have been tried.

Whereabouts are you?

[dhenry]

I am in Brighton.

I just drove the car to Devon and back this weekend and it performed like a champ other than the misfire. I either have to drive like a grandad below the misfire or put my foot in it. The misfire seems to go away at around 4500.

Let me know about those HS2's. I will be interested in a set although I am looking for a bargain as I have to split my time and money with the BMW.

Haven't tried the choke. Never used it. The cable is all a bit crusty and it doesn't really work too well. I will have a look though and I am certain it is off as i checked this when I rebuilt the carb a few months ago.

[dhenry]

My mistake, the choke cable is fine and the choke works perfectly. It is the heater cable that sucks. Tried the choke last night and on the way in. Didn't get a really good run because of traffic but the one time I did get up to speed and felt the misfire I pulled the choke out and the engine smoothed out and began to accelerate. So It would appear to be running lean at that point.

I am still learning and I am more familiar with weber style carbs so bear with me. Could it be related to the needle or might there be something else that is work within the carb? If it is the carb why would it only be causing trouble when in 4th? Cheers,

[vulcanbb18]

Is there oil in the dashpot damper of the carb? It sounds like there isn't!

What carb is it? Hs2 or HS4? It would be worth at least taking the dashpot and float-bowl covers off and having a good look. Perhaps it is a carb off another engine and has the wrong needle.

Don't be put-off by the SU, they are great carbs (I like them much more than webers), they just work a little differently than most

cheers

Jacob

[dhenry]

I like SU's too. Just don't know them inside and out like webers. There is oil in it but I do suspect that it came off of a different engine at some point. I will investigate further. Cheers,

Daniel

[robert]

daniel , there are a few things that jump to mind ,

1) ht leads , have you changed these ? well worth it ,especially in this rainy season .

2) coil terminals , have a really close look at the connections to the coil,sometimes green mankyness ,can build up in the wire connection to the spade on the end of the wire .if in doubt cut them back to fresh copper and put new ends on .

3) is the engine earth lead to subframe all good ,and the earth lead from the battery to the body clean.

4)if its going a bit lean at mid range , try turning the nut under the carb ,just above where the fuel pipe from the float chamber fits into the black plastic bit ,looking down at it ,you turn it clockwize for richer ., try a turning it one quarter turn and see how that goes .

5)definately change the plugs .

regards
robert