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Request for advice - Rapide 276 Carburettors

BlackLightning998

Well Known and Active Website User
VOC Member
Afternoon all,

I'd be grateful for any advice Forum Members can offer with a strange carb problem that I have just picked up.

The bike is a C Rapide, 12 volt Pazon Ignition and recently snapped a front carb throttle cable at the throttle end.

The bike was running fine before, it has air filters push fit onto the carb bodies.

The bike has unusual 276's on it, bolt on flange from a bike other than a Vincent but they have been nicely done and nicely set up - I have done 2800 miles on it with just John Coates doing a carb tune for me and it running like a different bike after that.

When the cable snapped I ran it home about 10 miles at 30/40 miles per hour with the front cylinder running on tickover once the cable snapped.

So, what I've done:

1 - Tank off
2 - Carb top off, slide and needle out to disconnect old cable
3 - Make up new cable and fit, replace slide/needle unit
4 - Carb top back on as before
5 - Replaced rear carb throttle cable at same time
6 - Refitted tank
7 - Sync'd the throttle opening on the cable adjusters
8 - Changed the oiled front plug - running NGK 7's
9 - Went for a run and found the jet body nut loose and petrol around the base of the carb body (I hadn't touched it that I know of - maybe nudged the float chamber and that has done the deed?)- tightened.
10 - Went out today - 10 miles into a run the front plug fouled and bike went onto one cylinder - running very rich when I took plug out.
11 - Hadn't touched the settings on the slide or air on the carb that I know of.
12 -Tried opening the air screw - didn't seem to make much difference as it was the one and a half out at least. Started the bike - ran on one cylinder again.
13 - Whilst riding along, took off the air filter on the way home - seemed to make a huge difference (weakened mixture I guess by allowing more air to flow and bike ran smoother as a twin all the way home.
14 - Whilst riding and pulling plug lead off the rear cylinder, when I open the throttle the bike bogs down, I can look down and see petrol spitting out the front carb bellmouth.
15 - When I do it the other way around, the rear carb pulls cleanly.
16 - Plug still black and sooty when I got home (30 miles).

What is puzzling me is that everything was working fine before and now I have it running very rich to the point it fouls the plug and goes onto one cylinder.

The only other work I have been doing is replace the dynamo which unfortunately fried itself because I had not checked what bulb was in the headlight when I bought the bike and it transpires that it has a big 55/60watt halogen thing - mmmm - one nicely cooked dynamo which threw commutator segments.

On the carbs and rich running - has anyone come across something similar before and can point me in the direction of anything to look at?

Much appreciated all.

Regards

Stuart
 
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john998

Well Known and Active Website User
VOC Member
Carb problems

Hello,
Only two thought come to mind, Have you got the slide needle back in the correct groove, if so I would remove the carb and make sure that the main jet is in place.
I have ridden my twin with a cable broken, but feel it is hard on the motor.
Regards John.
 

BlackLightning998

Well Known and Active Website User
VOC Member
Hi John

Hi John,

I didn't disturb the needle, which is in the second slot (which I think is the "standard" setting to the extent there is one) - interestingly the needle in the rear carb is one slot lower.

I suspect that taking the carb off and having a good look, clean, blow and wipe is the best solution - the bike was running fine before and now not - so surely only the things I have touched could have caused this fault?

I wouldn't have thought I have done any lasting damage running slowly for 10 miles on one cylinder do you?

I know now that removing the plug is the preferred option when doing this next time though.

Thanks for taking time to reply - perplexing....

Stuart
 

Len Matthews

Well Known and Active Website User
VOC Member
It is possible to drop the slide in back to front,i.e with the cutaway facing fowards. I know, I did once and wondered why the poor old thing was running so rich. Peering up the bellmouth soon revealed the reason!:)
 

BlackLightning998

Well Known and Active Website User
VOC Member
I ran in the garage to check that - no, it's right

Hi Len,

Many thanks. I ran in the garage to check and no, the cutaway is facing rearwards to the bellmouth of the carb. The hunt continues.

Cheers

Stuart

It is possible to drop the slide in back to front,i.e with the cutaway facing fowards. I know, I did once and wondered why the poor old thing was running so rich. Peering up the bellmouth soon revealed the reason!:)
 

BlackLightning998

Well Known and Active Website User
VOC Member
What a great excuse to stop polishing.....

Hi Ossie,

I'll have a check this evening and see all is OK - I think it should be fine as the bike has push on air filters which have been there since I bought the bike.

Thanks for your thoughts.

Stuart

make sure the holes at the bellmouth intake are clear had mine full of solvol polish too much cleaning over the years.
 

BlackLightning998

Well Known and Active Website User
VOC Member
Latest update - any more thoughts?

Evening all,

Here is the latest update for consideration and any more advice please. I've tried to answer some of the advice I've had embedded in this update.

I torqued the heads down myself a couple of months ago, there is no sign of anything blowing out, they are as clean as a whistle and I have good compression - so I think that there is a good seal (although I don't think I have the modified gaskets fitted!!).

I stripped and cleaned the float bowl assembly - all spotless and back together again, level right and does not flood.

I went back to the original plugs that were in the bike when I got it, a 5 in the rear pot and a 6 in the front. BUT this time I swapped them to see if I could get it to clear and put the 5 in the front. I left the rear on the NGL 7 that I've been running for 200 mils or so. I thought that the 5 would run hotter and help it clear itself (of all the muck I may have left in there by running for 10 miles on a snapped throttle cable with the plug left in and connected).

I spoke to Pazon (ignition manufacturers) to see if it was possible I had damaged the electronic ignition system with what I have been doing (lifting a plug lead) - they were very helpful and confirmed that on electronic ignition you should never lift the plug lead without grounding it as the electrical spike has no-where to go and may find its way back into the control module, they suggested a range of checks which I did, the final one being to swap the ignition leads over and run the front cylinder with the rear cylinder lead - I did this and the bike ran fine in the garage with no difference which way round the leads ran - so seems OK.

Then I took the bike out tonight for a 50 miles run. Results as:

I did the first 20 on the M40 Westwards at a steady 50-60 mph with the bike running clean. When warm, some lengthy bursts up to 80+ to blow it through. I felt it was doing slightly more work on the rear cylinder so I stopped when I pulled off the Motorway and adjusted the throttle cable slightly and it ran much better, feeling smoother and running beautifully well balanced and pulling strongly.

I stopped and filled up with fuel - unleaded just like normal and from a garage that I have used before. I then did 20 miles of A and B roads – ran perfectly, actually better than I can recall for some time, just tootling along in 4th at 40mph rolling on and off the throttle – very, very smooth, sounded superb exhause note and if I shut the air slide on either cylinder I choked the engine, open it again and it pulled clean.

Last 10 miles were in light traffic on the route back home – 30/40mph – by the time I got home running lumpy again, on and off the front cylinder and when I pulled the plug at home it is as black and sooty as it has been once again.

The rear cylinder I left a NGK 7 in it, looks fine when I pull the plug, slightly black outer edge and the central section mid-brown.

Next I think a proper strip and clean of the front carb is called for, so at least I know that has been done and can be discounted.

Any other thoughts or advice welcome?

Stuart
 

Len Matthews

Well Known and Active Website User
VOC Member
Although not related to Stuart's on-going problem, I pondered for ages as to why the Mk2 Concentrics on my bike seemed very rich at small throttle openings. Getting a reasonable tickover was impossible. First thoughs centred around the cold start plungers. Were they seating correctly? They were, so that's not the reason. Pilot jets? Nothing wrong there. Slide cutaway? No, not that. One thing I had never done was unscrew the Main Jet holders from the Carb bodies so I took them out. As there is no sealing washer between the hexagon and the carb body and as the threads are quite coarse, it occured to me that fuel may be sucked up past these threads so I put the jet holders back in with PTFE tape and Lo! much improved low speed carburation!
 
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BlackLightning998

Well Known and Active Website User
VOC Member
Try cobottom

Hi Len,

coarse won't work, but cobottom may - although it doesn't quite describe threads that are not fine but you know what I mean....

Anyone got any clues on my on-going carb problem (see earlier posts) - I'm stuck.....

Stuart

Although not related to Stuart's on-going problem, I pondered for ages as to why the Mk2 Concentrics on my bike seemed very rich at small throttle openings. Getting a reasonable tickover was impossible. First thoughs centred around the cold start plungers. Were they seating correctly? They were, so that's not the reason. Pilot jets? Nothing wrong there. Slide cutaway? No, not that. One thing I had never done was unscrew the Main Jet holders from the Carb bodies so I took them out. As there is no sealing washer between the hexagon and the carb body and as the threads are quite coarse, it occured to me that fuel may be sucked up past these threads so I put the jet holders back in with PTFE tape and Lo! much improved low speed carburation!
 
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MartinRestorer

Website User
VOC Member
slide needle

Hi Len,

coarse won't work, but cobottom may - although it doesn't quite describe threads that are not fine but you know what I mean....

Anyone got any clues on my on-going carb problem (see earlier posts) - I'm stuck.....

Stuart

Shouldnt your slide needle settings be the same, ie lower as your rear carb
is set. Also are you sure petrol isnt getting by your float needle.
O and thanks for advice on crank balanceing [JJ engineering]
Martin
 
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BlackLightning998

Well Known and Active Website User
VOC Member
Hi Martin, glad JJ worked for you---

Hi Martin,

Thanks for the reply.

I have cleaned and polished and checked the seating and sealing of the float needle - all OK.

Everything I have read says that you need to set the carbs up to match the bike and there is nothing which says that the settings or jets need to be the same from one carb to the other, they just need to work properly.

I am thinking of dropping the needle one notch to try - but I have had other advice saying that there may be an oiling problem, either the rocker feed bolt or (he thinks) more likely the scraper/oil control ring failure.

I'm hoping not - still working my way through the problems....

Stuart
 

Don Morris

Active Website User
VOC Member
Nothing to add regarding the problem, but a point earlier about running with a broken throttle cable. This happened to me and what worked well was to set slow running adjustment high on the broken cable carb. and set the other one as low as you can.

This leaves you with a bike that manages to idle and as you open the throttle the carbs balance out at quite a respectable speed.
 

vin998

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Hi Stuart,

You state the plug is fowling up when running but are not sure if it's oiling up or running rich. In my experience, if the plug is very black but a sooty dry appearance then it is running too rich and is due to carbaration. If the plug is very black but wet appearance then it is oiling up due to worn valve guides or pistion / rings. Hope this helps.

Simon.
 

BlackLightning998

Well Known and Active Website User
VOC Member
Sooty, black but dry.

Hi Simon,

Many thanks for your interest and reply.

The plugs are coming out very black, sooty but dry. My own reading and research suggests rich running as you say, although I have had a coupld of pieces of advice about oiling up - one being a failed scraper ring and the other the age old discussion point of rocker feed bolts - mine don't have metering wires but the engine has been rebuilt with new valves, guides, in fact everything from the pistons upwards as well as all new primary and timing sides. There is a possibility of a problem but it will be a component misassembly or failure issue rather than wear I think.

So I do hope that you are right - here is what I plan to do....

I am leaning towards carburation as the problem - and that is where I am going to focus my attention next.

Although my father has always brought me up to change one element at a time to resolve problems I think I have enough information now to do the first four at one go and then try the bike:

1 - Spark Plug choice
I have done a bit of research on spark plug choice and intend to try some NGK platinum plugs as well as some Denso ones - both with an projected electrode which will run slightly hotter than the unprojected Champion N5C. I don't think I will try Iridium plugs just yet.
My plug advice is saying:
"Denso ZU platinum options would be W20EP-ZU (projected) or W20ES-ZU (non projected), these are non resistorised plugs. You may be able to use an Iridium plug with your electronic ignition, perhaps Denso IW20 (projected).

If you are persistently fouling plugs the problem is more likely to be carburation but you may be able to work around the problem with a warmer plug, IW16 or W16EP-ZU. N5C is a non projected plug and therefore will have the effect of running a little cooler than BP6ES for instance"


I had thought that the NGK BP6ES was a direct swap for the Champion N5C but this would seem to suggest that the NGK will run hotter and so would be a better bet. The Denso W20EP-ZU looks like the Denso weapon of choice to try.

2 - Clean the carb thoroughly
I am going to combine this with stripping the carb completely (jet block out)and giving it a thorough clean and inspection and reassembly.

3 - Drop the needle one notch (then 2)
I'm going to drop the needle by a notch to start with.

4 - Redex
I'm going to add a dose of Redex for carbs to the petrol.

Then I'm going to go for a good long run (weather permitting) and keep my fingers crossed.

After that, if it does not solve the problem, I'll put the needle back to where I started and look at firstly jet size and then cutaway size to follow as I follow the AMAL tuning notes guide and seek to sort out the richness I think I have.

If all that fails - then I think I am heading towards exploring the oiling up suggestion.

Thanks again for your interest Simon, it's much appreciated - I'm finding the problem very frustrating and as you can imagine it is spoiling my enjoyment of the bike.

I'll keep the Forum posted.

Regards

Stuart

Hi Stuart,

You state the plug is fowling up when running but are not sure if it's oiling up or running rich. In my experience, if the plug is very black but a sooty dry appearance then it is running too rich and is due to carbaration. If the plug is very black but wet appearance then it is oiling up due to worn valve guides or pistion / rings. Hope this helps.

Simon.
 
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