Crossflow running probs and exhaust heat shield

[robert green]

Just taken my 1700ss out for a blat. I noticed on my last run that it was missing a little, but it now seems to being doing it a lot more. Basically starts fine and runs normally until you try and use more than about half throttle at any revs. It then splutters and misses a fair bit. I have read the carbs need balancing etc regularly. The car was serviced by redline 2 months ago, and I have since covered about 2000 miles. Is this the likely cause, or is it more likely to be the ignition system?

 

Also, my heat shield on the exhaust system has come loose where it is welded to the silencer and now rattles horribly. Can this be refixed, or is it a new silencer?

 

Any help for a newbie appreciated!

[Anthony]

Balencing the carbs is quite straight forward. Suspect the poor running/misfire is electrical though. Might be worth checking first

 

You can balence carbs when the elec's are fixed

 

Regards

[John E]

Missing and spitting sounds like carbs to me but even if its been serviced its also possibly a plug going off so I'd focus upon that and then balance the carbs. Check that there isnt a blocked jet in each carb. Easty to do if you do one at a time. Depending upon the age of the dizzy its also possible that there is a points issue.

The shield is held on by large jubilee clips that pass around the silencer and thro sleeves on the shield. These are held on by poxy tack welds and I suspect that yours have broken. Best to take off the shield and either get a new one, about 20 quid I think or get them re welded. You do not need a new silencer.

 

run baby run

[Harvey]

Timing?

[Roger King]

Carburettor inbalance has less effect at large throttle openings that at small ones, so it is unlikely to be this.

 

Ignition system has to work harder under load than at light throttle, so it could be igniton related.

 

Carbs do sound likely though as others have suggested. A blocked main jet could be the culprit, but check also that one of the locking screws that hold the aux vents in place hasn't dropped out. These screws (with locnut) are loctated underneath the carbs at approx 4 o'clock and 6 o'clock depending on which cylinder. There is one screw per barrel so it's easy to see if one is different to the others. If this screw is missing, the aux vent can turn and block the main jet fuel supply to that cylinder. Light throttle running will be unaffected.

[Midnight]

When re-attaching your exhaust heat shield it is best to only tighten the jubilee clips enough to stop the shield from moving. If you tighten them too much it tends to pull the spotwelds/channels away from the inside of the shield.

[J.R.]

Bet it is what Roger King suggested, had the same problem myself, exactly the same symptoms.

 

You will also probably see fuel dribbling form out of the retaining screw hole.

[Mr Locust]

Pump jet linkages on dellortos can touch the top of the dizzy giving interesting results.

Also - anyone who has experienced a petrol leak onto the dizzy and got away with it is unlikey to let it happen again!

[John Hughes]

Had a similar prob myself last night - started to misfire / pop and bang through exhaust on acceleration above about 3500-4000 - thought the worst but luckily it was as simple as the plug lead had popped off No. 3 plug.

 

Hope your prob is as simple !!

 

John

[robert green]

Thanks everyone for your tips and advice. I started with cleaning the plugs up, which helped, but didn't cure it completely. The plugs did look like the original ones fitted though, so now 8 yrs old. A new set from Caterham has cured it completely and she is running sweetly again - just in time for 300 miles in the sunshine this weekend! Thanks all - and the heat shield welds had gone - a new loosely fitted one has done the trick.