Flat top carb rebuild

Brum

Forum User
Hi all, haven't worked on carbs for about 15 years and looking for detailed instructions on rebuilding flat tops. Anyone know of a good instructional manual or video?

Thanks
 

Ian Patmore

Well-Known Forum User
Brum,
These carbs have got a bad reputation, "boat anchors" phrase attached to them. Mainly as they are more difficult to tune, but if they are understood, they can be better than the Hitachi HJG46W 240z carb. eE.g, the flat top carb, HMB46W has a power valve, which puts more fuel in at a certain high rpm. These carbs like others, only have three items to meter, fuel, air and vaccum, how hard is it?

Not many fans of these carbs here (I am one), there is a thread on classiczcars.com which has some info. There is good info in the Nissan workshop sevice manual covering the L20A to L28 engine, green cover. There was a few Nissan sevice up-dates for the North American market, as they had "problems". Getting and keeping these to factory adjustment is key. Spares, very hard to come by, sometimes see a service kit on Fleabay, I have a few bits.

I would say the HMB airbox is better than the HJG one, as Nissan changed the design, which to me makes sense.

Get the manual/understand the carbs first, I will help where I can.
 

Brum

Forum User
Brum,
These carbs have got a bad reputation, "boat anchors" phrase attached to them. Mainly as they are more difficult to tune, but if they are understood, they can be better than the Hitachi HJG46W 240z carb. eE.g, the flat top carb, HMB46W has a power valve, which puts more fuel in at a certain high rpm. These carbs like others, only have three items to meter, fuel, air and vaccum, how hard is it?

Not many fans of these carbs here (I am one), there is a thread on classiczcars.com which has some info. There is good info in the Nissan workshop sevice manual covering the L20A to L28 engine, green cover. There was a few Nissan sevice up-dates for the North American market, as they had "problems". Getting and keeping these to factory adjustment is key. Spares, very hard to come by, sometimes see a service kit on Fleabay, I have a few bits.

I would say the HMB airbox is better than the HJG one, as Nissan changed the design, which to me makes sense.

Get the manual/understand the carbs first, I will help where I can.

Thanks Ian. I'm going to give it a shot and start stripping them.

Will reach out once I've hit my first hurdle
 

Farmer42

Club Member
The problem with all carbs of that age is wear. The rebuild kits from the states are expensive and places like ZTherapy won't touch them. You could go to the expense of buying the kits costing upwards of £200 only to find the throttle shafts and bushes are worn and they won't run. You probably won't get hold of those bits.

They may have been better than the 240z round tops when they were factory fresh and tuned but 40 years on they will be nowhere near.

I would recommend considering the HS6 or HS8 SUs where you can get all the bits from the UK and are reasonably priced. Otherwise get some round tops from ZTherapy.
 

Ian Patmore

Well-Known Forum User
I agree Farmer 42, but 40-50 year old round tops will be nowhere near either factory fresh, unless rebuilt.

Some like to use the original equipment supplied with the car, new, rather than going with the easy, in-correct route. Just because ZTherapy won't touch them, doesn't mean they can't be rebuilt. There was a refurb kit on Fleabay Uk not long ago...

Brum, some useful info on Classiczcars, like this;

https://www.classiczcars.com/forums/topic/60309-were-bringin-back-the-flat-tops/#comments
 
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