427 misfire

[Bigsieuk]

My 427 has started to develop a bad misfire to the point where it barely runs on four cylinders when it gets hot. I have run a Crane Cams XR i breakerless ignition module for about 10 years and I'm wondering whether the heat has killed it. I'm going back to new points, condenser, ballast resistor and coil to try to eliminate the problem. Also the vacuum advance was seized in the distributor too so I'll replace that too.

Anything else I should look at?

Thanks all,

John

 

[Roscobbc]

First thing I'd consider is replacement of HT leads.

 

[Oneball]

I’d do one thing at a time. So leads or coil first.
Carefully put your hand on the coil when it has been misfiring and the engine is off and see if it’s really hot. If it is I’d do that first.
You can also look in the engine bay at night with the engine running and see if you can see any sparks from the HT leads or dissy cap.

 

[Roscobbc]

A 'no cost' inspection of the distributor cap interior is worthwhile. I have an MSD system and distributor on mine - there must have been a batch of dodgy rotors used for a while 'cos I had a couple with insufficient pressure on the centre 'wiper' where it rests against the centre carbon HT contact in the distributor cap - it consequently arc'd and eventually destroyed the cap. Look for telltale lines of carbon on the underside of the car - could indicate a 'path' for HT to short across. Also if running a high output system check that non of the plug caps are arcing against the head, metal exhaust gaskets or headers - best look in the dark for that.

 

[Bigsieuk]

Thanks all,
The coil was very hot when on the car. I replaced it with a spare one I had but the spare may have been one I kept but should have thrown out. I have bought new one which I'll fit this weekend and hopefully try it out (weather forecast is thunderstorms though).

Existing cap looked fine but I bought a new one just in case.

I have the plug leads with the outside sheathing - what are peoples views on these, I thought they might hide HT leads shorting?

 

[Roscobbc]

Thanks all,
The coil was very hot when on the car. I replaced it with a spare one I had but the spare may have been one I kept but should have thrown out. I have bought new one which I'll fit this weekend and hopefully try it out (weather forecast is thunderstorms though).

Existing cap looked fine but I bought a new one just in case.

I have the plug leads with the outside sheathing - what are peoples views on these, I thought they might hide HT leads shorting?

Mine had the stainless sheathing on when I first bought it. Not sure whether they were old (didn't seem to be) or were of inferior contruction but they were certainly an issue in terms of getting a good reliable spark. Any issues like a little bit of flooding or a fouled plug and an individual cylinder wouldn't fire. Just my experience - quality OEM spec' sheathed leads mau well be fine.

 

[Bigsieuk]

Thanks,

These are the second set of sheathed leads I've put on in 20+ years of owning the car. I've tried to keep the original look but they don't seem to last as long as good quality non-sheathed leads.

 

[Roscobbc]

For me, personally all the ignition shielding stuff, like oem leads, distributor in a 'can' all to minimise radio interference was a waste - yes, fine for perhaps old school medium and long wave radio reception......but FM - not really. The issues with mine were the weak spark due to the sheathed leads creating a 'stumble' off idle and further-up the rev range. The addition of a multi-spark MSD ignition system totally transformed the low speed running of the engine - stock L36 manual would 'pull' from 7 or 8 hundred rpm and 10/12 mph in 4 th gear.......such flexibility.......for a while - untill the increased spark started 'jumping' from one spark plug boot to the cylinder head (too close).
Before doing too much I suggest trying the car out on the street at night - without the engine hood. Not just at low speed but under load where it will be more likely to have potential shorting issues. That spark has to be going somewhere - if not down the HT leads perhaps elsewhere. Best to remember that electricity takes the least path of resistance. High energy ignition even more so......

 

[Bigsieuk]

Thanks Rosco,

I'll get into the garage this weekend and see if I can sort this out for once and for all.

 

[Bigsieuk]

As a recap the engine would misfire when hot and eventually wouldn't start, I was running a Crane Fireball ignition module in place of points, which I assumed had given up, so I replaced it but it still wouldn't start.

I chased power from the battery to the coil from the starter and the 'run' source from the ignition, both tested fine.
Ballast resistor was fine, coil wasn't providing that big a spark from the HT lead so I put on a new coil - still no start.

Out with the replacement ignition module (Fast XR-i) and back in with points, with a nice spark at the points. Started immediately.

Sought a refund on the XR-i (bought through eBay) with little hope of a refund only to get a response the following day with a "not worth paying the postage so have a refund". Common quality issue?

I'd like to have known how to test one of these units and avoid time tracing the faults and replacing unnecessary parts.

Anyway its running again

 

[Daytona Vette]

Quote: "As a recap the engine would misfire when hot and eventually wouldn't start"
That bit of the post alone would suggest; hot engine and fuel vaporisation - Ethanol issues

 

[Bigsieuk]

Good point. I haven't driven it in heat since (weather has been rubbish). Finger crossed.

Whats the solution if it is?

John

 

[Daytona Vette]

Good point. I haven't driven it in heat since (weather has been rubbish). Finger crossed.

Whats the solution if it is?

John

Hi John
Use Esso Synergy fuel (less ethanol in the mix - but not in Cornwall and some areas, look the areas up on the Esso site) use a heat shield between the carb and the inlet manifold,
You need to do this anyway as Fuel Vaporisation is common with C2s and C3s especially big blocks and the fuel companies are going to increase the percentage of ethanol in our fuels

just as another point imo do not go back to contact breaker points when there are electronic points around such as Petronix - much smoother pick up and acceleration
Good Luck

 

[CaptainK]

As above, get a heat insulating gasket that goes between the carb and intake manifold. I used to get dreadful hot starting issues, eventually worked it out (newbie mechanic here), as one day I stopped the engine and wanted to check something under the bonnet. So got out and opened immediately to hear the sound of a coffee percolator under the bonnet. A quick Google later (other search systems are available) and I learnt all about heat soak and fuel percolation. I fitted the heat insulation gasket myself, and I'm a newb mechanic, so it must be easy

 

[Bigsieuk]

Thanks guys,

Are the Petronix pieces better quality/different manufacturer than Fast XR-1?

Are there heat shields for 6X2's?

John

 

[Daytona Vette]

I only have knowledge of Petronix and it replaced my twin set of mechanical contact breakers on an Accel distributor with good results

Speak to Real Steel for your Petronix and your Heat insulating Gasket, they should put you right and be able to supply your parts as they did for me as long as you can tell them what Carb and Dizzy you have

http://www.realsteel.co.uk

Good Luck

 

[Bigsieuk]

Tri-power Carburetors & Related Accessories - Hot Rod Carburetors - Custom Tri-Power Carburetors - (Powered by CubeCart)

hotrodcarbs.com

 

[Roscobbc]

Before going too far with the heat insulating spacer thingy - best check if you have the available height under the hood - it can be rather tight.........

 

[Bigsieuk]

Yes, they mention the difference between 67 and 68-69 tri power manifolds on their website. I'll check before I order for certain. They sell a 1/4" set for 67's and a 1/2" for 68-69.