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Wrong Head/carb?


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#1 Gregmaton


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Posted 05 March 2019 - 08:59 PM

Help and advice please if possible.

I have a 1979 850cc. which has always had running difficulties I pulled the head and fully refurbed (new valves stem seals, ground valves in etc). I noticed that the head is a CAM4180 (998 head?) running an SU HS4.

Once put back together the engine will turn over but not firing. Spark plugs are getting wet (from fuel), Valve clearances adjusted correctly, timing correct and spark plugs are new and firing in correct order. Engine will run spraying easy start into the inlet manifold with the Carb off. 

Is it okay to run this 998cc head and/or HS4 or could this be causing the initial running issues. If not what could be the issues with the carb and should i rebuild the HS4 or should I swap to an 850 cylinder head and HS2? 



#2 wile e coyote

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Posted 05 March 2019 - 09:29 PM

Head carb combo is fine  - (1 1/16" inlets and 1" exhausts) the last of the 850's were fitted with HS4's......did the carb come with your 850?  - you've got fuel, air  and spark and as it runs with easystart  my bet is it's down to the carb - adjustment or needle - if carb hasn't been apart though I'd strip it down and make sure its spotless / fully functional and that inlet air leaks have been discounted

#3 sledgehammer


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Posted 05 March 2019 - 09:35 PM

First check fuel  -  use fresh fuel if in doubt


put fuel in clear container , look for water at the bottom / cloudy fuel


if all ok , check float is working ok & not overfilling wetting the plugs before they have a chance to fire


(this is rare , as usually it will fire a bit before flooding - but seen stranger things over the years)


next compression check


best of luck

Edited by sledgehammer, 05 March 2019 - 09:40 PM.

#4 Gregmaton


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Posted 05 March 2019 - 10:16 PM

Yes the carb came with the 850, previous owner didn't mention the larger head so should imagine he was unaware too!

 Forgot to mention, compression was fine but will try fuel and float as suggested. If that doesn't work I shall rebuild as recommended, doesn't look like its been cleaned/apart in a while ... Thanks for you help chaps

#5 viz139


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Posted 05 March 2019 - 10:36 PM

I had a Mk 1 850 brought to me were the previous owner had fitted a later 998 unleaded head, the compression had dropped to about 7:1 so was even less powerfull than the original. It ran but very hard to start cold and was very slugish.

#6 richmondclassicsnorthwales


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Posted 06 March 2019 - 01:58 AM

Sounds as though this was a bit of a project car you bought ?


Fuel is the biggy. If it is old fuel, it wont run. 


Years ago, the old 4 star lived forever, fuel these days is crap - simple


You may need to flush out the old fuel from the line and tank. Sledgehammer has said what to do primarily. 


Believe me - old fuel will not run an engine.  You need the 3 fundamentals - fuel - air and a spark

#7 Retroman


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Posted 06 March 2019 - 09:13 AM

As above the head and carb are not the issue, it sounds like its short of fuel


The float lid needle might be stuck up, or some crap behind it.


They can be prone to sticking shut and open....even the same day


A new float lid needle valve may well cure it

#8 Cooperman


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Posted 06 March 2019 - 09:14 PM

Apart from anything else, you need to measure and check that the compression ratio is OK. If you have just fitted a 998 head to an 850 block, crank & pistons, the C.R. may well be very low. It is an easy mistake to make and easily put right after measuring and calculating.

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