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Images have now been updated. Any ideas would be great. Thanks

Hey pal, I currently have almost the exact same problem with my H1, so far I've checked alot of likely causes although they haven't fixed it, it could be whats wrong with yours.

- Run the engine with the tank off, and spray carb cleaner or gt 40 or a similar flammable aerosol all around your carb inlets, airbox, the rubber manifold on the engine and the rubber vacuum pipes underneath the manifold. Any change in engine running - revs increasing or decreasing suddenly indicates an air leak.
- Thoroughly clean your carb and pilot jets, get them sonicated if you can (i cleaned mine with canned cleaner and it's still no good so im hoping sonication with do the trick)
- Remove all the carb components and check the condition of them as in the guide at the top of this section.
- Make sure your float heights are set right.

The 'melted' bits on the airbox in your picture are just from people putting the airbox on without seating them properly, they get bent out of shape when it's tightened up, it probably won't fix your problem but its a good idea to sort them out as it will run better later. Heat them up with a hairdryer to soften them up then pull them out of the airbox, then boil some water in a pan and submerse them in it for a minute. when you take them out they'll be really soft. Stick them on the carb inlet immediately and as they cool and harden up, they should be back to the correct shape. When you put them back in the airbox and onto the carb take extra care that they're properly seated before you tighten it up, lubricating them with a bit of silicone helps.

damn that's good advice!

just to add to the above, replacement is still the best thing you can do.

as the rubbers harden they also shrink, so your 32mm venturi is actually 26mm in the rubber...not ideal.

Aye, I wanted to replace them too, but it comes to almost 100 quid for a set of 4 from kwak!

Lozza excellent little guide there, but yesterday I actually won a whole airbox on ebay which has not been melted and looks in very good condition. Yes after looking at where they fit on I do agree with you that it probably isn't the cause of my issue but was worth replacing. I have just removed the carbs off the bike now so Im going to check what the carbs have been set up to. This should be interesting as my bike was running a beet nassert slip on can and the rectangle restrictor on the top of the airbox was missing so im guessing someones been messing about with air mixtures on the bike before.

Not completely sure on this but will be finding out while stripping carbs down for cleaning and re-setting. Then to finish up with balancing the carbs.

OK so this weekend just gone I had the chance to finally strip the carbs down.
After stripping them down everything seems fine. There were no blockages and no splits in the diaphram. However when stripping them down I checked all the settings while putting them back together.

Current Settings H Manual 89 H1 Model
Main Jet 102 98 102
Needle N36Y N77C N36Y
Pilot Jet 35 35 35
Pilot Screw 3 Turns 2 1/4 Turns 1 1/2 Turns

I found out the information for the 89 model H1 which is my correct model on this link done by mori man
viewtopic.php?f=55&t=10692&p=89062

So I am now going to change my pilot screw back in to 1 1/2 turns out and see if this has any affect.

My new airbox arrived safe and sound and is in perfect condition compared to mine. It also has a restrictor pipe in the top of the airbox which my bike doesnt currently have. No harm in putting it back to standard as I am only running a Beet Nassert Slip on Can.

Then I will rebalance and see how it goes.

OK so this weekend just gone I had the chance to finally strip the carbs down.
After stripping them down everything seems fine. There were no blockages and no splits in the diaphram. However when stripping them down I checked all the settings while putting them back together.

Main Jet
Current Setting = 102
H Manual = 98
89 H1 Model = 102

Needle
Current Setting = N36Y
H Manual = N77C
89 H1 Model = N36Y

Pilot Jet
Current Setting = 35
H Manual = 35
89 H1 Model = 35

Pilot Screw
Current Setting = 3 Turns
H Manual = 2 1/4 Turns
89 H1 Model = 1 1/2 Turns

I found out the information for the 89 model H1 which is my correct model on this link done by mori man
viewtopic.php?f=55&t=10692&p=89062

According to the sheet on the link just above suggests the H manual settings are for the H2 model.

So I am now going to change my pilot screw back in to 1 1/2 turns out and see if this has any affect.

My new airbox arrived safe and sound and is in perfect condition compared to mine. It also has a restrictor pipe in the top of the airbox which my bike doesnt currently have. No harm in putting it back to standard as I am only running a Beet Nassert Slip on Can.

Then I will rebalance and see how it goes.

Quite a big step forward this weekend. Put the carbs back on the bike and set up my auxilary fuel tank. Took a little while to get going but fired up lovely.

Once the bike had warmed up I then balanced the carbs and they were nearly perfect before I adjusted them anyway. Since changing the air screw from 3 turns to 1 and 1/2 turns. The revving seems so much better but wont be completely sure till I get her out on the open road.

Once the bike was warmed up and all balanced. I thought I would lozzas advice a go and see if there was any air leaks around the rubber manifolds between the carb and the engine. I sprayed some carb cleaner to cylinder 1's manifold and no change but on 2 , 3 and 4 all revved up to and just past 4k revs!!

So the bike now has air leaks on 3 of the cylinders lol. Not the ideal solution but at least I know what the problem is now and can start looking for replacements.

Thanks for everyones help.

At least you know for sure what the problem is now mate eh, I always find diagnosis the hardest bit.

Yes indeed. Upon closer inspection it appears that 2 of the 3 leaks is at the base of the mount by the engine aswell as the carb side and on the other one its just at the carb side.

Took them all off and there was two seal strips which are all fell off. Not unreasonable considering the rubbers are 25 years old now . Has everyone just simply bought replacements? or tried to repair them? as there are no cracks in the rubbers and look in reasonable shape.

If you mean that the o ring between the rubber and the enginehas fallen off/ apart then either get new o-rings and back them up with some silicone