HPBCANADA.COM

The thought it best to start a thread on this topic. I for one of many thought that the 60 hp and 70 hp carbs were all the same smaller size. When PPR [Professional Powerboat Racing] made a rule change in FV class to allow the 2000 model 56 oval port in the class it had to run with its original carbs. That same rule still applies in T850 for any oval port. None of the OMC racers back then ever ran the oval port block in the class. One racer purchased a new set of 2000 model 70 hp carbs and ran them on his SST60 powerhead. They did not work. When we see these carbs listed on EBAY they normally say they are for both 60 and 70 hp motors. I have never seen the 70 hp larger bore carbs. Craig Fraser has a complete 70 hp oval port powerhead?? Dave Elsey has seen the carbs on it and they are larger. I have a fresh 30 over oval port with carbs. They are the small bore carbs. We need to get the part numbers off Craig's carbs and we need to test those carbs on an oval port. It could be that the larger carbs were on some but not all 70 hp oval port models. OR we just have not seen a real 70 hp oval port.
Many T850 racers put their oval ports on the shelve because they were not aware of the larger carbs. Others who are building fresh powerheads will rethink what block to use provided the oval port performs.

REMEMBER THE RULES FOR THIS MOTOR
- it must run the original carbs
- you CANNOT port this motor I has to be be STOCK ported! BUT you CAN blueprint and balance the powerhead.
- 160 lb compression

Finally, do not assume at this point that it is OK to buy the larger carbs off the 2 cylinder model and run them on your 3 cylinder oval port. Inspectors will have the correct part numbers and specs for the original 70 hp carbs.

We may have found our first hint.
In 1992 OMC had two different carbs for the 70 hp model.
Early and late
Here is the parts list:
http://www.boats.net/parts/search/BRP/E ... parts.html

This can really change everything.
PROVIDED the early carbs are big bore older style carbs.
Those carbs can be run on the oval port!!

Now when we go to 1995 there are two different carbs again.
http://www.boats.net/parts/search/BRP/J ... parts.html


1998 has one carb for all three
http://www.boats.net/parts/search/BRP/J ... parts.html
it was used up to 2001
http://www.boats.net/parts/detail/brp/B-0439452.html

carbs on mine today have a stamped number on the side that is 339172, no idea what this links to, but also a D1 on the carb under this number.
yes they are larger. these carbs came from a 1993 70hp.
a lot of 70hp engines seem to have the 60hp carbs on them, I think these could be from 1997 on ? no idea as to why this happens.

here is the size I have measured today.
bore of carb back and front is 1 1/2 inches exactly,
the venturi on these carbs is (from what my 2 metal rulers and guage show) 1 inch and 31 /100ths. so what that is in fractions i leave to you.
nearest I make it is 1 5/16ths venturi. maybe you see it different ? let me know if you do.
I can only go by my measuring device and a metal rule both showing exactly the same.
these are todays pics showing what the measuring devices show .


.
.

My 1994, 70 HP had 1 1/2" throat but I never measured the venturi and sold the boat, motor, trailer late last summer, so I guess I can't help here. I've notice that the orifices size changes a lot from year to year on the parts catalogue, I wonder why.

The parts book shows the the main jet sizes are:
1992 model...50D
1995 models...51D and 52D
1998 and up..52D
Based on the posts above. This could be the carb??
http://www.ebay.ca/itm/60-hp-70-hp-JOHN ... 19fa6cf268

I read on several other sites problems related to the oval port. I think??? I now know what happened. The reeds and reed stops have always been the same on all the model years. Initially in 1992-1993 there were two problems with the oval port. One they had idle problems and second they blew up sooner than expected. Both problems must have been caused by the motor running too lean. To correct the problem OMC increased the jet size from a 50D to a 52D and then they reduced the carburetor size down to approximately 1 inch bore size. This increased the fuel to air mixture. The motor needs to run FAT!. All the motors were rated at 70 prop shaft HP and owners like the performance . It was better than the bridge port model.

In 2013 and 2014 we saw two oval ports in T850. Both had problems. The racers did not say much as to the reason[s]. Both were running the large bridge port carbs. I think in Jake's case he was running too lean at Gravenhurst. At Dunnville he ran 79D jets. Most bridge port SST60 motors run 73D jets maximum. A stock 56 bridge port runs jets around 69D. That is a lot of fuel going into that motor. If it can burn it, it makes a lot of power! It did. That motor on Dave Elsey's boat ran 77 MPH on GPS.

So would the original oval port carbs work? I think so. The larger one is what I would use. For a T850 race boat it does not matter if it has to idle at 1000 to 1300 rpm instead of 700 rpm. A 56D or 57D jet would be a good starting point. The motor will lose a bit in top end with the smaller carbs, but should make up for that shortfall with acceleration.

that's good to know Houndog
I couldn't find any info on the jets of different years.

Dont forget this engine also came as a 50hp .

And what would the letters D1 and D2 mean on the carbs ? the only difference I could see was the machining on the back face.
the D2 is a nice smooth machined finish. the D1 is a rough finnish. very strange.
on my larger carbs they are all D1 =rough finish .

They are likely casing numbers same as the other casing numbers on the carb. OMC likely used the same basic carb on a number of different motors. A single and/or twin carb smaller hp motors may use the same carb with different jetting. When they talk carb assembly as a part. That normally means that is the complete carb with the correct jets for that model of motor. The only difference between carbs with totally different part assembly numbers can be just the jets that are in the carb everything else is exactly the same. Comparing the parts in the parts book will tell you if there is any difference in the internal parts carb to carb.
The earlier larger bore carbs would be the way to go for a T850. The newer smaller bore carbs would be the way to go if you had a pleasure boat application. The first carb used on the oval port did not have the plastic top. It did have air/fuel adjustment screws on it.
Both the plastic top and the non plastic top carbs are legal in T850. The racer would include that information on their motor spec sheet.