Missfire under heavy load

M

Majicmark

New Member
Can anyone throw a new light on my running fault?
My 2200sc starts &drives fine on short or flat runs of any distance,the problem comes after it has been pulling up hill over a long drag & noticeable in any gear but more prominent in 3rd or 4th.
The car starts to loose power slightly & gives the impression of a misfire ,but not on one cylinder is seems to be across the whole system. Once you level out the car sorts itself out & drives fine again for any distance until another hill is reached.
As I live in Cornwall these are hard to avoid.
I have the feeling it is electrical breakdown of some sort & have done the following .
I replaced the head gasket to cure the usual oil leak ( although fault existed long before this). The valve clearances were checked & reshimmed then.
I found the plugs were over heating so have replaced them with another standard set & then a cooler set to try.
I have replaced the coil,condenser ( several times), points, Dist cap, rotor arm,HT leads,LT lead,earth in dist,base plate & fly weight springs in dist. I have now fitted an Accu spark ignition system & wired to fuse box ,reset timing,re- checked valve timing & still the problem occurs. Today coming back from a show I was stuck in traffic aprox 40 mph & it never missed a beat even up hill but under light load. It only occurs under heavy load & feels like a condenser does when it's failing but have not got one now.
Any ideas?
I am running out of ideas
Thanks Mark
 
A friend had a similar problem, it was the rotor arm, which was new! We drove it until it had the same issues as you described. Scratched hour heads and then replaced the rotor (had seen some posts saying some new ones were bad) with another new one. Problem solved.

I have also been running the Accu spark on my 2000auto for a couple of years now and I love it. Transformed the car.
 
richarduk said:
A friend had a similar problem, it was the rotor arm, which was new! We drove it until it had the same issues as you described. Scratched hour heads and then replaced the rotor (had seen some posts saying some new ones were bad) with another new one. Problem solved.
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Similar story on my MGC
 
could also be coil - they often misbehave when warm as can condensers if you're still on points...
 
Majicmark said:
The car starts to loose power slightly & gives the impression of a misfire ,but not on one cylinder is seems to be across the whole system. Once you level out the car sorts itself out & drives fine again for any distance until another hill is reached.
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My first thought was that this was fuel starvation. This would also partly explain overheating plugs. Have you had any pinking/knocking? Starvation shouldn't be an issue with you single HIF carburettor - they are far less susceptible to slosh than the HS6 type, and even then the carb would flood uphill and starve downhill.
Hmmm, have you checked the float level recently? If it is set too low (ie, proud of the chamber walls) it could cause starvation with extreme incline changes. If you've not done it before, you need to remove the carb to check, but it's not that hard - just the 4 nuts. The float should be set so that you can place a rule across the centre of the chamber and have the centre of the curve on the float just kiss it (+/- 1mm or so). Bend the brass tab to adjust. You need to remove the jet for this (the screw with the spring on it and bimetallic right angle bracket), but don't worry, this won't upset the mixture as long as you don't touch the mixture screw.

Just a thought, but worth exploring the ignition side as well.

Michael
 
Tried another coil,rotor arm,leads & dist cap tonight off of my 2000 Auto & still the same.
Redrover I also thought it maybe time to look down the fuel route & was hoping someone may second the view ,
I have an electric pump on order anyway but will take carb off & check float level,(I had overlooked that one).
It is as you say the HIF carb & had been messed with badly when I bought the car. The needle was mis aligned & jet was rubbing on the side. Replaced the needle with the same that was in there but does anyone know what number jet should be in it? I wonder if that is wrong too!
 
Seems to me that you have pretty well eliminated the electrical side of things now, Mark. Just for completeness, have you verified that the vacuum advance is working OK by sucking on the tube and watching the distributor base plate rotate?

But then I do agree that it is carburettor time. Don't mess around with it, simply buy a full overhaul kit including needle and jet and do the lot. HIF carbs (which yours is) do produce strange results when the needle and jet are worn. So this could well sort things. Plus having everythiing apart is also likely to reveal any of other problems I can think of in this general area!

Best of luck

Chris
 
chrisyork said:
Plus having everythiing apart is also likely to reveal any of other problems I can think of in this general area!
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Yes, that's right. I've had a weekend of it trying to diagnose a lack of power in my TC (which I have fitted with a pair of your HIF type carbs). The fuel filter jumped off inside the pump glass bowl, and I had to chase a blockage around the system. Problem was, it kept moving! I had the carbs in bits (again! :( ) by the end of Sunday. It's only been 1700 miles since I rebuilt the whole engine!
They are very clever things, and able to compensate for a lot of variables (which HS's can't), but the price for this sophistication is (as always) much greater complexity!

Michael
 
A tired or simply loose in its mounting fuel pump could cause fuel starvation when the demands are high like the situations you describe.
 
Since you've done a number on the ignition side then I can think of three possibilities:

1. Check your valve gaps. Lack of power under heavy load can be due to a few gaps that have closed up since your cylinder head overhaul. [At the same time, check your camshaft timing as per the excellent thread and photos elsewhere on this site.]

2. Fuel starvation. Check carb float levels, any fuel filters present and the fuel pump delivery [disconnect the fuel pipe, run it into a container and run the engine - don't rely on the manual priming lever on the pump and obviously take fire precautions seriously]. It's possible that some dirt was dislodged from one of the float chambers during the cylinder head overhaul and this is now blocking one of the jets - this happened to me and it was a pain to finally track down and eliminate. A Barry Cade is to loosen the accelerator connection between the two carbs and then rev each carb individually - if one carb in particular tends to spit back or stumble then you've narrowed it down. Check the basic mixture setting - it's surprising how often one believes the SU is well tuned [after going through all the Gunson carb balancer, lift pin and Colour tune rigmarole and getting it all spot-on] only to find that the carb is actually on the lean side when it comes to road testing. Only a tiny, tiny turn on the mixture adjustment screw can be the difference between a smooth runner and a stumbling dog.

3. Inlet manifold air leak. One possibility is the O rings in the inlet manifold - especially if you disturbed them when you had the carbs and manifold off. [Hint: If you do replace the O rings, put a washer under each carb-to-manifold bolt - this will allow you to snug them up and compress the new O rings nicely and evenly. Smearing the new O-rings with something like 3 Bond before installation is also a belt and braces suggestion.]
 
I found the plugs were over heating so have replaced them with another standard set & then a cooler set to try.
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Plugs might be running hot due to a slightly weak mixture. Weak mixture can cause hesitation under heavy load.

If you go to http://www.sucarb.co.uk/ProductSearch.aspx and enter your SU carb part number (which shoud be stamped on a small metal tag somewhere on your carb), the parts list available for your carb should show you the standard needle for your type. Maybe one possibility is that you have a needle that is too weak at fuller throttle?
 
My 2.2 is in storage, but funnily enough I had that exact problem last year with my S-type Jag. First of all it was only a slight misfire when I'd been caning it, and it stopped when I moderated a bit. Then it gradually got worse and forced me to abide by the speed limit on the motorways. Horrible concept. And then finally it got so bad it acted up every time I depressed the throttle going up a long hill. I went through the whole bloody thing stem to stern - thought it was head gasket (ok I'm paranoid, but it had overheated badly just before this started getting noticeable), compression check and leak down no problem. Then I thought maybe its just the coil. Changed that. Nope. Ignition amp I robbed off a scrap Series III XJ6. Nope. Maybe the HT/LT leads. Nope. Ah, thinks I, must be those electric SU pumps. Theres nothing else left. They're original, must be a bit knackered by now. Nope. Maybe an airleak through the intake manifold gaskets or a carb seal. Nope. At this point I'm getting stressed because I've dismantled, rebuilt, or replaced half the greasy parts on the car and I cant drive it over 50 and I cant drive it up hill. And its my daily driver. And my wife is nagging me to buy a Honda. Then finally I notice that big glass bowl with a fuel filter sitting in it... turns out the fuel tanks are a bit rusty, and the fuel filter was just about clogged solid - fine for running around town or a quick burst of overtaking, but any time it had a consistent load on it there wasn't enough flow through the filter and the engine would starve. So the new filter cost me one pound something or other, and by that point I'd changed about 300 quids worth of parts and spent about 50 hours trying to find the problem. Moral: look for simple stuff first. Hopefully it'll be that simple on your P6 (but without the 300 pound expedition first!) Good luck.
 
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