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can someone please explain the 56 cu in jet theory to me.
https://hpbc.ca/viewtopic.php?f=19&t=3392		 Page 1 of 4
Author:  phillnjack [ June 5th, 2016, 6:45 pm ]
Post subject:  can someone please explain the 56 cu in jet theory to me.
In my daft quest for power and speed I have changed carbs from the 60hp set to the bigger 70hp ones.
the larger carbs are from same type of engine (oval port 56 cu in) and both sets of carbs are the black plastic tops.
this is supposed to be the only difference between a 60hp and a 70hp on the later 56 cu in oval port engines.

Now the jetting on these engines to me seems a bit weird.

on the 1995 year 60hp carbs the jets are 42D for the main (in the bottom of carb) and 35D for what I presume is air at top of carb
(small bore carb).


on the 70hp carbs for 1994 year the jets are 53D for the main (in the bottom of carb) and 60 for what I presume is air at top of carb.
(large bore carb) notice larger main jet than the following years ?.


on the 70hp carbs for 1995 year and onwards the jets are 52D for the main (in the bottom of carb) and 60D for what I presume is air at top of carb.
(large bore carb)


on the SST60 the mains are 73D and the top (again I presume to be air) 33 ...
(large bore carb).

what I cant seem to understand is the 60hp has larger mains than air, and so does the sst60, yet the 70hp has larger air
jet than the main jet ?
I am totally confused by this, I was on the understanding that mains are always a bit larger than the air on these engines.

IF the reason behind this is the 70hp carbs having larger venturi letting in more air, then how come the sst60 has larger man jet than air jet ?

please can someone explain this to me.
Author:  hec2buck [ June 7th, 2016, 10:18 am ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
:) Yes, the reason the plastic top carbs have smaller hi speed jets (52D's) is due to the supplemental fuel entering through the intermediate circuit (60D's). The metal carbs (large bore) from the SST-60 have no intermediate circuit and run 73D's for the hi speed.. The 33 air bleeds on the metal carbs are the fixed low speed air bleeds. The plastic top carbs have adjustable low speed needle valves that are set around 2-2 1/2 turns out.

Keep in mind the air bleeds work opposite fuel metering jets. For Fuel jets the larger jet flows more fuel. For air bleeds the larger jet flows more air and leans down the mixture.

Hope that helps.
Author:  phillnjack [ June 7th, 2016, 7:29 pm ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
ahh now I get it.
that makes perfect sense now.

thanks hec2buck
Author:  phillnjack [ June 8th, 2016, 7:24 pm ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
on the larger (new to me ) 1993 70hp black plastic top carbs ive just noticed the jets in these from new are 53d main and
55d on the top jet.

on my year engine the 70hp carbs are the same apart from the jet numbers, mine are 52d main and 60d for top !!!
what difference could this make ? would the smaller top 55d make the engine run a bit leaner ?
Author:  hec2buck [ June 9th, 2016, 8:51 am ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
The difference is probably due to the smaller venturi & throat in the later model carbs. The smaller venturi has a higher pressure drop and sucks in more fuel than the larger venturi, so a bigger jet is required. I think the older carb has 1 1/2" throat, while the newer carb has a 1 3/8" throat. Check it out.
Author:  phillnjack [ June 9th, 2016, 1:33 pm ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
on the later carbs for the 60hp the front of carb is 1.5 inch then it gets down to about 1 inch in the venturi and then
only enlarges to 1 1/4 inch at the back of the carb.
the 70hp carbs are 1.5 inch at the front and same at the back with a venturi a touch nigger than 1.25 inches
so its not the bore size of the carbs causing jet changes.
Author:  hec2buck [ June 9th, 2016, 1:56 pm ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
Yes, you just measured the later carbs to be smaller. The inlet will be the same on both. The venturi is smaller on the later carbs ( 1" vs. 1.25" ), and the outlet to the reeds is 1.5" on the early carbs and 1.25" on the later carbs. So you require less hi speed jet size as I stated earlier.
Author:  phillnjack [ June 9th, 2016, 4:37 pm ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
No that is not correct.

the inlet and outlet on 70hp carbs is 1.5 inch both ends with 1.25 venturi. that is both early and later carbs.

that is the only genuine size 70hp carbs on the oval port engines listed from 1993 - 2001.

the 60hp carbs have 1.5 inch front opening and 1.25 rear (reed end) with 1 inch venturi, the 50hp carbs are even a touch smaller.

none of the carbs with smaller 1.25 inch opening at the reed end were intended for the 70hp engines.
the carb bodies for the 70hp are totally different part numbers.
its not just the jets.
Author:  Hevi Kevi [ June 10th, 2016, 9:36 am ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
phillnjack wrote:
No that is not correct.

the inlet and outlet on 70hp carbs is 1.5 inch both ends with 1.25 venturi. that is both early and later carbs.

that is the only genuine size 70hp carbs on the oval port engines listed from 1993 - 2001.

the 60hp carbs have 1.5 inch front opening and 1.25 rear (reed end) with 1 inch venturi, the 50hp carbs are even a touch smaller.

none of the carbs with smaller 1.25 inch opening at the reed end were intended for the 70hp engines.
the carb bodies for the 70hp are totally different part numbers.
its not just the jets.


So the later 70 carbs have a 1 1/4" venturi now? I thought you said they were 1 5/16" and really hard to find? Changing your story now?
Author:  LittleCharger [ June 10th, 2016, 10:04 am ]
Post subject:  Re: can someone please explain the 56 cu in jet theory to me
Hevi Kevi wrote:
phillnjack wrote:
No that is not correct.

the inlet and outlet on 70hp carbs is 1.5 inch both ends with 1.25 venturi. that is both early and later carbs.

that is the only genuine size 70hp carbs on the oval port engines listed from 1993 - 2001.

the 60hp carbs have 1.5 inch front opening and 1.25 rear (reed end) with 1 inch venturi, the 50hp carbs are even a touch smaller.

none of the carbs with smaller 1.25 inch opening at the reed end were intended for the 70hp engines.
the carb bodies for the 70hp are totally different part numbers.
its not just the jets.


So the later 70 carbs have a 1 1/4" venturi now? I thought you said they were 1 5/16" and really hard to find? Changing your story now?


New BBQ tongs! (I still laugh at that one)
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